I have some duplicates in my customers. Merging them works fine, but for some of the customers I cant select the Primary I want. Its just not there as an alternative. Does anyone know why some customers dont seem compatible for merging ?

Are they inactive?

No, they are not inactive

Doe they have value in the Represents Subsidiary field?

👆 That's a good one I didn't think of. To expand on what Luis said, the Represents Subsidiary field being filled in means it's an interco customers and they have all sorts of restrictions on them that regular customers don't. It would make sense to me that merges wouldn't be possible for them.