It was a matter of mental discipline. Matoran do not have the mental discipline to activate masks, but I'm quite sure most of them know masks by their shape and power. Vakama of course would have to be very knowledgeable about this subject, being a mask maker. I think its possible that the suddenness and surprise of becoming Toa made it harder mentally to learn how to use their mask powers than most Toa. They were kind of created on short notice, after all.
That would make sense.... if all they had trouble with was the activation. But it's been repeated numerous times that they didn't know their powers at all.