They have to pay within 15-20 days (can't remember) from receiving all the required paperwork. The FMCSA wrote the regulations to balance between giving the carrier the resources to get all the documents they need to say compliant and the O/O getting paid in a timely fashion.
I would check with OODIA on what resources are available. If you are not a member this would be worth joining just to deal with this. They aren't going to after the carrier for you but likely have a letter that is written by an attorney that you can send.