Stale claim

I consulted in December 2019 for the first time, since joining Momentum Health4me. The Optometrist that I went to, submitted a wrong claim code. Momentum did not pay. And I only learned recently that they did not pay, which means I owe the Optometrist. When I enquired about it, Momentum maintained that the claim won't still be paid because it is now considered stale. But who should pay? I'm not the one that made a mistake but why do I now have to settle the debt whereas I pay Momentum every month?