Are you sure you were close to a REM sleep stage?
FILD only works if you are doing it while a REM dream is about to form.
If your Alarm woke you up in the middle of a N-REM, you won't find yourself in a FA (so a REM dream) immediately.

So the question you have to ask yourself is "ok my Alarm went off, was I just dreaming?'
If yes, you have chances to return to a Dream through FILD, if not, it will probably take much more time than 15s