Mission Control Stopped Working on MacBookPro

I have just recently noticed (so I can't pin it down to anything specific), that mission control (Control Up-arrow) doesn't execute any more. Neither does Control-Down Arrow. However the Control- Right/Left arrows work fine to switch spaces.

I pretty much have the same kinds of things installed on my MacBook Pro that I do on my iMac, and the latter is working just fine.

Running OSX Yosemite.

Any ideas on what I can do?

Thanks,

Korky