I've also found that if I waitlist for a points+copay upgrade, I won't see offers. It'd be nice to see them because I would likely grab one if it was competitively priced with the value of the points+$ upgrade. I'm guessing this is by design, though. They'd rather me burn the points and give them some cash in the process.
I tended to think this was a strategy, but I'm really leaning toward it just being crap tech at this point. Once ANYTHING changes on the initial itinerary, the offers go away. One of my itineraries changed by 7 minutes and now no offer.