Check that there is a publication date in the catalog entry for the PDF. DOIs can also only be registered if they have a publication date.
How do I know if my DOIs have errors or failures?
The registration agency you use should provide you with regular error reports which describe broken DOIs.
If you have a journal which changes the URL syntax, a best practice is to re-register all DOIs for that journal to ensure the URLs are up-to-date with the registration authority.