I have had the exact same experience. Nightmare -- every charge results in denial, denial, denial. Resubmit, resubmit, resubmit. and calls to company.
I submitted a 700 receipt for my portion of a crown, along with dental receipt. It's very clear the amount is what I paid. They still deny. Now they want an EOB from Dental Insurance Company. This has been going on for 5 months and I'm no longer with dental insurance provider.
Why do we have to jump through hoops to get our money? I have a repeated service performed - finally get one claim approved and they continue to deny every one after even though supporting documentation is EXACTLY the same. Every charge (other than prescriptions) results in me having to resubmit documentation multiple times, then having to make a few calls.
----- Next year, I will not be participating in flex spending. It's just way to hard to use it.