Apple pay using Apple credit card - Transactions declined or not...

Hello,

Lately, I encountered some transactions being declined using Apple Pay.

I read threads about "usual banks" credit cards being declined when using Apple Pay. But in my case, I am talking about using my own Apple Mastercard card through Apple Pay.

For example, on August 10, I paid $19.44 at 6 pm in a grocery store and then at 6:30 pm I could NOT pay $11.76 in a Target store. I had around $5,000 card balance!

Today, on August 12 I could NOT pay $434.42 at Pottery Barn. I had over $5,200 card balance!

The processing device said "Card declined". This is frustrating and embarrassing.

This is not the first time this happened in about two months. Each time, I was in my home State, at most about 45 km from home. I was not in China or Russia or whatever...

What am I supposed to do to fix this nonsense?

Thanks in advance for any help/explanation.

Regards,