Won't pay claims and won't tell you why

I have had this coverage for 2-3 years. Up until recently, we only had to use it once a year for an annual exam. (We purchased the rider so that routine care is covered.) They paid those "cheap" claims easily. Unfortunately three months ago my dog got critically sick and we spent several days and several hundred dollars in emergency vet costs. To date they have not officially responded in any way to the 4 claim submissions. They won't pay and they won't even tell me why. I have called six times but have been left on hold so long that I gave up waiting for a human. I have tried emailing but they won't respond with any claim-specific information. They will confirm receipt of the claims, but will not say anything beyond everything is still "under review". Their customer service is beyond awful. Once we get this mess behind us, I'm cancelling my coverage. It isn't worth it.