There is a CPU thread where you can find plenty of examples.

I myself scored a bunch of CPUs at the window just in the summer on flights between EWR and ROC. Yes, I know exactly what you are describing, where there are plenty of open seats and no upgrades have been handed out, it's called UA wanting to get more money. UA trots out 737-700 for Rochester on the late evening flight and then first flight out back in the morning for maximum fleet utilization and bottom line, except F is not going to be sold out for a 40 minute flight. UA still delays upgrade in some cases because UA is arrogant.