I'd say mine don't post about 25% of the time. There's not always a pattern too it, but I will say that it's most common on itineraries where there's been a change somewhere since booking. For example, if I originally had an itinerary of Thursday to Sunday, but then switch to fly home on Saturday and do a paid upgrade on those flights, the upgrade PQPs are less likely to post on their own (even though I paid for them after I changed the itinerary). No idea why that seems to be the case.