Hi there,
Jack from OpenThinking here!
That’s probably because the Facebook catalogue isn’t syncing with your Instagram one correctly. I did find this guide in the Facebook help centre that has helped a few others get this issue sorted. You can manually add the products to your Instagram catalogue if they are not appearing and then you will be able to tag them in your Instagram posts. They also have a ‘troubleshoot catalogue’ page in the help centre here too that may help.
let me know if this works for your. If yes; accept my answer and give me thumbs up! Thank you.