Maybe it is due to the fact that CO and NW take out "F" inventory shortly before EUA runs (at your upgrade window), and then replace that inventory after the "automatic" upgrade window closes. This allows people that want to use miles the opportunity to use them for upgrade, and also leaves more first class seats available for sale.

That being said, I have seen CO do this many times, but never NW. Be thankful that you can actually call and manually upgrade with NW (as OnePass will not upgrade you manually even if the inventory is there).