Difficult to align the lower incisors

I have been working on an 18 years old boy on the finishing stage. His lower front teeth have remained crowded for the whole length of the ortho treatment (2 years). I have tried repositions, different wires, IPR, etc. Not much improvement obtained. I am wondering what might be the reason for these lower front teeth being so stubborn? The case is a class 3 tendency case. Very limited overjet and tight overbite present. Could this be the cause?