See title. I found that most western nations tend to be somewhat reluctant to name the events in 1915 by what they were.
Is it solely because they don't want to hurt their relations with Turkey, or is there an actual dispute whether or not the documents and reports etc. are true?
Short answer: A little bit of both, but mostly the former.
There is some problems in international law with retroactively labeling the Armenian Massacres of 1915-16 as a "genocide".
The concept of genocide in international law, as distinct from other kinds of mass murder, only was defined in 1948 with the adoption by the United Nations of the Convention on the Prevention and Punishment of the Crime of Genocide. /u/tayaravaknin can probably explain this in greater detail than I can, frankly.
Since very technically genocide was not a defined crime in international law until 1948, to label the events of 1915 as "genocide" would be applying a retroactive judgement to events in terms that might not have been understood that way at the time.
Collocquially, we are free to label the 1915 Massacres as "genocide", but to ask a government to do so is rather more difficult.
There is also some debate as to whether the modern Republic of Turkey is in fact a legitimate successor state to the Ottoman Empire. In fact polities by each of those names fought each other in the complicated set of conflicts termed the "Turkish War of Independence".