For retreatment of endodontically restored tooth, no complication only on radiograph appear short, you reopen but failure to go apically, no stick sound

For retreatment of endodontically restored tooth, no complication only on radiograph appear short, you reopen but failure to go apically, no stick sound

What is the cause of failure to go apically: 

1. Perforation

2. Separate instrument XXXXXXXX

3. Mud

4. Ledge