I've never had this problem before but it came up after I moved from a Windows 7 development machine to using Ubuntu 14.04.
I didn't notice anything amis until a client asked me to put Captcha on their site. I got all my files of our git repository and a dump of the database so I felt that I had everything I needed but strangely "captcha" and "freeform:captcha" were not responding at all. In fact "if captcha" and "if freeform:captcha" branching always resulted in false!
Specifically I was using Freeform but the problem was in my setup not Freeform
It's got to be in the configuration so here are the things I checked:
I've listed all these things to check as it could be handy for debugging this problem if you come across it, however none of these things solved my problem.
Just in case I tried to install this php library
We have Captcha!