Unfortunately, TurboTax has a limit on how many t-slips can be imported that is less than the amount that the CRA can send. There's also a limit per slip type, so if you have a lot of T5008 slips, that is likely why you are not able to Auto-Fill.
The limit should be indicated at the point when TurboTax is trying to import the CRA data. Would've saved me almost an hour of trying to find a way in which it would work.