Why won't our 4 week old baby finch fly?

My breeding pair of Zebra Finches had a successful clutch of three this time around, and while two seem to be thriving (leaving the nest, learning to eat on their own, etc) the third refused to leave the nest. The mother and father have both tried pushing him out to no avail, so I reached in to bump him out, this is when I discovered that he cannot fly. Straight to the bottom of the cage he went, and this is where he sits for 10 hours a day. Mom and dad feed him, but when he attempts to fly, his wings spread out to his sides and he falls over, as if his legs can't support him. Any thoughts as to what might be going on here?