So did Slavs migrate with the Germanic, Scythian/Sarmatian/Alan, and Hunnic tribes during the Migration Era? It's remarkable to me that we don't hear anything about the Slavs or slavic lands from the Romans.
The Slavs actually do get a lot of attention, but most of it's not being written in English. Like all other studies of 'barbarian' migrations, it's something of a sticky issue - modern political agendas (the desire to root modern national identities in ancient people groups) have resulted in some pretty dreadful scholarship which tries to find Slavs in times and places where they weren't, in order to provide historical evidence for modern debates about ethnicity and national identity in the former Soviet Block (see Geary, The Myth of Nations, for a scholarly rant about this scholarship). If anything, many historians have been far too quick to identify archaeological finds as 'Slavic' (I'm thinking particularly of 'Slavic' pottery in Greece, which almost certainly had nothing to do with the Slavs).
The book you should read if you want to learn more about Slavs in the early middle ages is F. Curta, The Making of the Slavs: History and Archaeology of the Lower Danube Region, c.500-700 (2001).
Curta has published a followup to this book on his academia.edu page, which you can read (for free) here: https://www.academia.edu/422133/The_making_of_the_Slavs_Slavic_ethnogenesis_revisited . It's dense, but might be helpful.
We do, but a bit later. The sixth century Byzantine historian Jordanes discusses the Slavs in his works, and there are other mentions from around that time. It may simply because that the Slavic migration was "behind" the German one, and thus they did not come into major contact with the literate cultures of the time until later.