Claims not being paid

We are a medical practice and have been struggling since 30 December to get a payment from GEMS. the patient have already received his appliances and we have already paid our supplier. I have just spent an hour on the phone and were cut off. No one can give us any answers. Im fed up waiting for money. for 6 Months. The roblem is, because this has bee going on for longer than 4 months, the claim is now stale.... but this was not a fault on our side.