I've noticed that certain applications seem to trap the gestures, even when they're no longer active.
The only time I've been able to 100% narrow down something trapping various shortcut keys was with either Apple Remote Desktop (the built in screen sharing functions), or Microsoft Remote Desktop.
Even though I clicked outside of the app and started using something else, something caused notably my ctrl+←/→ to stop working.
I unfortunately do not remember the specific steps I took to resolve the issue. Whether it was explicitly sending focus into the remote connection window, and then (with the mouse only) explicitly out of the window, and onto another app that is not just the Desktop, might have fixed the problem. Otherwise, I may have just closed the window in frustration.
Either way, this issue is one that you may have to do some personal diagnosing to figure out. Check for any running apps that are notably not native (Java, AIR-based, etc.), or have some other form of deep integration (trapping keyboard events and sending them to another computer is pretty deep), that may be causing this issue.
I'm not sure that this really counts as an answer, but in my case the problem went away after I did two things:
1) installed the most recent update from apple, and
2) manually closed all my programs and restarted them (I usually just have them reopen automatically when logging back in)
Best Answer
If you are using Yosemite 10.10.2 beta, according to the developer it's due to a bug in OS X. Here is the tweet from the developer:
https://twitter.com/boastr_net/status/538498628500082689
You can also find a temporary fix there.