If your bills are getting rejected and not paid in a timely manner, it almost always boils down to two things: Poor or lack of communication and knowing how to submit your invoice correctly with the right line items.
In some cases you may never meet the adjuster handling your job in person. If you are lucky, you may talk once or twice on the phone. More often than not in this day and age an e-mail or two may be your only communication besides the invoice you send in at the end of the job.