I have the same problem, and it’s also because of the shift-key.
I think it’s because my laptop doesn’t register too many keys or whatever.
(at IcyLime) But don’t worry, the hard is easily earnable with that little problem. :)

I’m not sure which game in particular you may be referring to, but have you checked that Sticky Keys is off? I have experienced the general bug in the Multitask games of keys randomly not registering as pressed, or not holding and staying held. However, the fact that you actually said “It’s very frustrating to bash the shift key 5 times and seeing the pink thing happily fall down a hole” which is the exact key sequence generally used to activate Sticky Keys (hitting the Shift key 5 times in a row) makes me think that’s probably your issue.

I haven’t tried the game since April, so I wouldn’t know if it has been fixed, but I’ve used Chrome/FireFox since forever. And it can’t be sticky keys, since it was the only game I had problems with.

I see. Apologies.
Weird, though; I’ve got no problem at all and I wouldn’t be surprised if I had considering I’m always too lazy to update anything – unless, of course, I need it to play a game.
So, well, good luck with it. :p