I just wanna say, if you happen to be using a glue that is solvent based (I think that's the one) give it at least 24 hours. I rerooted a G4 scootaloo, glued the hair, gave it a few hours and popped the head back on. Later on in the evening I was taking some photos of her and noticed turning her head felt a bit gummy, figured it was the glue and it would sort itself out.
Little later on, the head was REALLY soft and squishy, as though it had been soaking in boiling water too long, and easily popped on and off the head. Eventually it went back to normal but it took weeks! D: