Login to Identity Portal. Go to Setup\CAIG. Confirm "User Matching attribute name"? This must match what is being used in IG. You can go to IM and review \Manage Users\find the user in question and select. For example.,
So if your mapping in IP (for user matching attribute name) is set to LoginID but the login for IG is userID, the certs would not show the mapping. Login to IG.
Go back to IP\Setup\CAIG and use the matching attribute. Ie., Using userID instead of loginID depending on what is being used for IG. If not matching this would cause the certifications to not be assigned as you are seeing.
Another common cause is simply the universe not being enabled. Login to IP. Check \Elements\Universe and confirm its enabled. In the details \Details\"Enable Universe to display certifications campaigns".