We had the same issue with Brickman. They refuse to pay for parking lot sweeping we did because they claim our insurance is not compliant. Even after a conference phone call with their compliance department assuring the A/P department that we were 100% compliant, we are still waiting on that check. Wrote it off a long time ago and don't do business with them anymore.
Most of the 3rd party companies are realizing that they can get away with crap like this- USM has rejected numerous invoices because they were "not in a format our system recognizes," for "incorrect amounts" when it's the exact same invoice from the month previous with the updated date, etc.
If they throw enough crap against the wall, eventually some of it sticks and the contractor just gives up or they can hold that payment longer and increase cash flow. Each invoice they don't pay goes right back into their pocket, and they know there's nothing the small companies can do about it.