IAD-DEN is very hard because inbound intl. pax in both F & C book into domestic F, there is a lot of paid Y/B (M for 1K) travel and YCA (govt) are all eligible for -UP at the time of booking and clear ahead of CPU's. Thus a Silver on a B fare clears ahead of a 1K on a <B fare.

DCA-DEN is only 1x/day and runs full. Lots of paid F.

This is a route where if you want to sit in FC, you will need to locate a viable FC fare or buy one of the -UP's.