Refusing to pay valid claims

I was a member of Momentum health for 3 years and after a relatively normal membership, they have refused for over 2 months to pay a claim for tests and scans done after I got a refferal from a Dr on their network. It has been tons of back and forth with their own telephone consultants claiming they don't know why the claim was rejected. I am so frustrated with this. I feel like I am being taken advantage of because my membership with them has ended.