They have to file paperwork with the necessary supporting documents. If they can’t, you have no idea why. Maybe it’s because they’re ineligible and in violation of immigration and visa restrictions. Maybe it’s because the documents were lost in a move. Maybe they had their identity stolen and are awaiting new documents. Maybe they just don’t actually give a shit about the job and aren’t bothering to do the paperwork to start working.

All you can say is that you’ve asked them to fill out the paperwork and they haven’t done so.