Tadpoles not growing into frogs

Hi
I've been looking at the replies here, but no one seems to know the
answer(including me).
I have the same problem. I've had my pond for four years plenty of frog
spawn but no frogs. I still have tadpoles in there from months ago. My
son says they are retarded and fit in well with the rest of the family.
I talk to them regularly and give encouragement but they seem happy as
they are, just chilling as they say. Is there any real reason for this
apart from just being lazy.
Sandra