Yes, there were several 'monotheist' movements in Hinduism, and depending on how you look at it, the religion itself can be seen as monotheist.

Examples given were about Buddha, Jain, Sikh and some more communities who pray to a single god.

Here I have to clarify, Buddhists and Jains don't pray to one god, nor any god really. Buddhists and Jains generally believe in the wheel of samsara, and have god-like beings which are considered worthy of veneration (Tirthankara in Jainism and Bodhisattva in Buddhism). However ultimately there is no 'One God' like the Christian or Muslim one, except for Indonesian Buddhists who believe in Sanghyang Adi Buddha as an ultimate god. However this is only because of the Indonesian state's policy of only recognizing Monotheist religions, so Buddhist thinkers had to find a 'one god' for their religion so it would be recognized.

Sikhs, however are monotheist, the concept of 'Ik Onkar'- One God, is central to Sikhism.

Now onto your main subject; Hindu monotheism.

Hindu theology and cosmology is characterized by various ideas from diverse thinkers across centuries. However, one main concept common to most schools is 'Brahman' also called 'Atman' and 'Paramatma'. (Note: Brahman does not refer to the caste. The caste is pronounced 'brahmin', while this is pronounced 'Brahmaan').

Brahman is considered to be the universe. It is an eternal and infinite spirit which is responsible for the creation of all the worlds and universes. The gods you are familiar with, such as Shiva, Vishnu, etc are all equally part of Brahman's creation as we humans, although on a higher plane of existence. In Hindu thought, Brahman created the universe from itself, and is the ultimate cause for everything, does not change yet causes all changes.

So Brahman sounds a lot like one god, right? So does this mean Hinduism is monotheist?
Well, not really. There's a reason why there is debate. Unlike the Abrahamic God, who is considered to be all knowing, often involving himself in human business, demands worship, and most importantly, is distinct from his creation, the universe, Brahman just is. This is complicated theology, so I'll try to explain easily.

The most important difference is that Brahman is not distinct from its creation. That is, everything is Brahman. One of the most famous Sanskrit religious phrases known by most Hindus is अहं ब्रह्म अस्मि (Aham Brahmasmi), meaning 'I am Brahman'. The world we live on and everything upon it is Brahman itself, and upon realising this ultimate truth (recieving divine conciousness), one's soul achieves moksha, that is, after death their soul will be reunited with Brahman rather than being resurrected. Brahman is said to have created the universe out of itself, a common metaphor is how we grow hair. Hair is from our own body, yet distinct, yet ultimately is is a part of us.

So Hinduism can be considered to have one god, but can you call a spirit that is the universe, and who isn't worshipped, as a god?

Now for more straightforward response, yes, there were and are monotheist movements in Hinduism.

Hinduism can largely be divided into 'Dvaita' (dualist) and 'Advaita' (non-dualist) schools. The Dvaita view, propagated by Madhvacharya, a c. 12th century monk from Karnaraka, considers Brahman to be a seperate thing from its creation, although they are ultimately identical. So you could consider them to be monotheist since they have a distinct idea of one god. In fact, some Dvaita schools considered Vishnu to be Brahman, and a compassionate, caring god, who would reward his devout worshippers, so they are indeed monotheists.

The most well known monotheist Hindu sects today are surely Arya Samaj and ISKCON (International Society for Krishna Consciousness) and Hare Krishna, as ISKCON is known in the West. Their conceptions of God are also more akin to the Abrahamic one.

Arya Samaj was founded in Bombay in 1875 by Swami Dayananda in an effort to make Hinduism a strongly organized religion and to offer Hindus a uniform education in their religion to combat colonial missionaries. The sect considers the Vedas to be the infaillable text of Hindu religion, and consider there to be only one god, referred to by the syllable 'Aum' (Om). They reject idol worship, and their worship consists of fire-worship and chanting of mantras, particularly the Gayatri Mantra. Further, having a strong core text and core ideals, it became a missionary religion, with a goal to lead all people to the light of the Vedas. It carried out all sorts of missionary processes to 'revert' Indian Muslims and Christians to Hinduism, and even set up branches, and gained local converts in countries with Indian diaspora, from America, to Kenya to Fiji.

ISKCON was founded in 1966 in New York by Swami Prabhupada as a successor of the 'Krishnaite' traditions, which considered Krishna to be the 'one true god', and considers the Bhagavad Gita and the Bhagavata Purana as core texts. Their worship consists of veneration of Krishna, and a follower is meant to think of Krishna as much as possible. They also sing songs in praise of Krishna publicly in a practice called Kirtan, which is supposed to serve the dual purpose of attracting new people to the sect. The practice of japa, chanting the name of Krishna 108 times, is also vital for salvation. ISKCON is also a missionary organization, with footholds in places without significant Indian populations, such as Ghana and Russia. However, ISKCON is considered somewhat cultish, and has been at the centre of numerous controversies from allegations of abuse, corruption, assassinations, etc, although that's a whole different answer.

In conclusion, yes, there are and were monotheist Hindu sects, and Hinduism itself can be considered monotheist depending on how you look at it, although it is distinct from the Christian monotheism you may be more familiar with.

SOURCES:

Chandogya Upanishad

Katha Upanishad

The Wisdom of the Vedas, by Chatterjee, Jagadish Chandra

Sources of Indian Tradition Volume One: From the Beginning to 1800, by Embree, Ainsley T.

Bhagavad Gita As it Is by AC Swami Prabhupada (for the foreword and appendices regarding ISKCON. This book is the ISKCON sanctioned version of the Gita)