Nice article Dr. Gary Brannigan (@GaryBrannigan) with 9 suggestions for helping a student with homework completion problems. #3 is especially pertinent to math:
Initially assign homework with which the struggling learner is unlikely to have difficulty. Mark the homework for punctual submission and content. Gradually increase difficulty but never beyond the struggling learner’s ability to succeed with moderate effort.
This is called shaping behavior. By starting slowly and moving a student towards the desired outcome. Simply getting the homework turned in on a regular basis is often a major task.
I also posted about notebooks. My approach is to have students use a simple folder for storing homework and to leave their notebook in class. The initial focus is on completing homework and submitting it on a regular basis. As the homework becomes more challenging the student will begin to take the notebook home.