Aetna denying claims bc they say there was no referral

Aetna has repeatedly denied claims for my infant’s care. My baby has seen the same pediatrician since the weeks she was born and a cardiologist who the pediatrician sent us to since only a few weeks later. She got vaccinations during regular appointments. For some reason a handful of cardiologist appointment claims were denied, as well as some of the vaccinations, and for all of them the reason is “no referral,” even though there was a referral for the cardiologist, and for the vaccines it was literally during a pediatrician appt with her pcp.

I’ve been sent to collections for some items that should have been covered. I’ve spent hours on the phone with Aetna. They say they’re looking into it, but last time they said that it only resolved a few items, and I keep getting new charges.

The medical centers I take my kid to say everything looks right on their end. They keep sending the bills back to Aetna for review.

Any advice on resolving this and making sure it doesn’t happen again?