Weird clipboard behavior on GNU/Linux (Fedora 20, Ubuntu). Why does pasting from buffer work every other time?

Question

linux

Good afternoon!
I don't know how to google properly. But I have this problem:
1. I select the text in the Firefox browser, press ctrl+c or ctrl+insert (it doesn't matter, because it doesn't affect the result).
2. I paste the text into the graphical terminal using the shift+insert combination.
3. At this stage, the text is either inserted or not.
Moreover, "RMB -> Paste" works.
At first I had it in all Ubuntu, starting from 12.04 to 14.04, then I switched to Fedora 20 (Mate) and exactly the same thing happens here.
Please tell me how to overcome this illogical behavior of the clipboard.
UPD:
Solution found:
Use CTRL+SHIFT+C , CTRL+SHIFT+V - always works.

Answer 1

Alas, it's not about the layout. Forgot to write about it in the question.
I try to do the same actions in the English layout, several times in a row - and always a different result.