My experience has been: Sun, Mon, Fri - even with a decently high fare class - U, W - I can't clear upgrades. Even had a B fare did not clear on 1/3.
Mid week / off peak - even L has good chance of clearing.
DCA flights - very hard no matter what class or when.
International feeder flights in/out of hubs - hard to impossible - and it seems that is what UA focuses on for smaller markets.
It is really a combination of all these things for me.