My best suggestion is just to turn the thing upside-down and check that there's no gunk in the gears, and if there's anything obviously wrong. I'd probably use white grease for lubrication, but that doesn't mean it's necessarily a good idea! ;-)
It's worth mentioning that - while I wouldn't say mine is malfunctioning - my angle adjustment has been less easy than the height adjustment since new. I think it's as much down to the differing mechanisms used as anything. You may find that there's nothing more you can do about it.