Wait-lists can be closed for any booking class, but are programatically closed when a certain criteria have been met (i.e. a certain number on wait-list, 100% of the time, or many other variables.)

Closed means closed...no more wait-lists will be accepted either a PA or a PB (priority or non-priority.)

This is not to say that inventory may not be reallocated if demand falls off, just that UA does not want to wait-list any more people and would rather see the person buy the next higher fare, or shop elsewhere. The incremental revenue to them at the low booking class is not worth the risk of buy down, or displacing higher revenue customers.

Check alternate routings, or check closer to dptr (provided you still meet the advanced purchase rules,) or buy a higher inventoried fare.