Q:

Why UltiSnips does not recognize my own snippets?

Currently I use YouCompleteMe in Vim, then I have installed UltiSnips and vim-snippets with vundler. UltiSnips and YouCompleteMe are compatible and work properly; however, the problem comes when I define my own snippet because these are not recognized.
I suspect that the problem comes when I use the UltiSnipsEdit function because it creates the UltiSnips folder in my home directory and not inside the .vim folder. For example, when I am working with R scripts and I use UltiSnipsEdit, it creates the r.snippets file inside /home/UltiSnips. Then I define a snippet just to test:
snippet test "Testing snip for R." 
# snipp is working
endsnippet

After saving the file, the snippet does not apper on the list of YouCompleteMe nor is expanded with :UltiSnipsExpandTrigger (<c-l> in my case).

A:

Adding let g:UltiSnipsSnippetDirectories=[$HOME.'/.vim/UltiSnips'] to the .vimrc file makes it works. 

A:

I am a newbie UltiSnipes user and I got the same issue. It is documented in the offcial documentation (see here and search for UltiSnips-snippet-search-path).
To give you a short and pragmatic solution, my own snippet folder is ~/.vim/myUltiSnips, and I added the following lines to my .vimrc
let g:UltiSnipsSnippetsDir="~/.vim/myUltiSnips"
let g:UltiSnipsSnippetDirectories = ['myUltiSnips']

So (in the first row) you declare you snippets dir with absolute path, then (in the second row) add it to the dirs that UltiSnips search for, but this time using path relative to Vim's runtimepath.
NOTA BENE My Configuration works for me, but, I do not use vim-snippets.
From official docs:

Snippet definition files are stored in snippet directories. A snippet
  directory must be a subdirectory of a directory defined in the 'runtimepath'
  option.
g:UltiSnipsSnippetsDir defines the directory private snippet definition files are stored in.
g:UltiSnipsSnippetDirectories defines the directories for looking for snippets. Do not mix up this variable with previous one.

A:

Lets say you have to directories:

~/.vim/snips : for your own snippets
~/.vim/bundle/vim-snippets/UltiSnips/ : for the vim-snippets

There are two variables involved:

g:UltiSnipsSnippetsDir : The directory for your own snippets
g:UltiSnipsSnippetDirectories : A list of directories used for snippets

Without defining those variables in your .vimrc g:UltiSnipsSnippetDirectories is set to UltiSnips (without the path). Apparently it is sufficient to just list the name of the final directory instead of the full path. 
let g:UltiSnipsSnippetsDir="~/.vim/snips"
let g:UltiSnipsSnippetDirectories=["snips", "UltiSnips"]

