Key fob locks but won't unlock

[Pingouin]

Thanks bobolinko for the reply!
Yep, I switched the battery with a spare one I had lying around and it sorted the problem. Strangely, my other fob didn't seem to work either (which led me to believe that it wasn't the battery since I never use the other fob), and it worked when I got back home tonight
Still strange that locking was working fine. Also, the "dead" battery I removed is still at about 80% charge, so be weary that if it seems fine, it might still have insufficient juice!