I ran into this same problem before. There could be two reasons:
1. If there are duplicate product images within other product groups or items out of stock. If you have less than 10 items after that, the group won't show up
2. *more likely your issue* There is a glitch and you'll have to reach out to Pinterest support to get it fixed.
This happened to me before and it was a glitch. Pinterest was eventually able to get it fixed so it's worth reaching out!
Sounds like this might be an issue on Pinterest's side. I've heard a few different people are having this same problem. If you haven't yet, try reaching out to Pinterest support. Hope this gets resolved soon!