I was having a problem in neovim where my UltiSnips snippets were working, I could expand them, however YouCompleteMe was not displaying their availability in the pop-up completion menu. Actually, a couple of the snippets showed up, but not most of them.
My problem was that UltiSnips was loading with Python 3 and YouCompleteMe is on Python 2. To fix the problem, I simply told UltiSnips to use Python 2:
Now, YouCompleteMe populates the completion menu with all the matching snippets.