Stuck buttons seem to be a minor fit and finish issue.

I've noticed that it's possible to misalign the Mk. II faceplate far enough, that the buttons will stick. It was most noticeable on the down and right buttons. The jamming may not be noticable until the fasteners are tightened.

Misalignment may also cause the top edges of the handle to rub against the faceplate.

Glenn

PS. Tony, How about a FAQ or this.

The fix is to loosen the screws, nudge the faceplate, then retighten. Not too tight! Just enough to keep the cover from moving around.