NFT Collections Not Appearing

I create NFTs, and I’ve minted several collections.
My collectors are complaining that not all of my collections are visible in the MetaMask.
They receive an error when attempting to input the collection’s address & number:

Ooops! Something happened. You are not the owner of this collectible, so you can’t add it.

They do own the NFT, and I have tried it with my own copies, and receive the same error. These collections and NFTs were minted on Rarible. Any assistance would be appreciated.