I'm kinda of the mind that they were Clones for a while, but eventually, Palpatine got tired of spending th money producing more and started to force the people of various worlds into his servitude, so while there are still some of the original clones as storm troopers (ie: the head whacking), there are also a lot of guys who are Joe Blow off the street, who don't want to be there.
It would explain why they are so skilled in EP II, and can't hit the broad side of a barn with an ion cannon by EP IV.