Lower incisors that are very difficult to do leveling and alignments. Any comments?

This is a rare case among my regular braces patients. I have been working on an 18-year-old boy on the finishing stage. His lower incisors have remained crowded during ortho treatment (for 2 years). I tried bracket repositions, different archwires, IPR, etc. The crowdings are persistent. I am wondering what might be the reason for this? This is a class 3 tendency case. Very limited overjet and deep overbite present. Any suggestions?