Do you buy your tix at the last minute? Do you then check in just before flight time? CO starts upgrading (theoretically) 5 days out. They do usually hold a number of F seats until a little while before departure. If you are doing things last-minute then it's quite likely that CO has already filled the F cabin before you purchase your ticket or check in.

Based on the changes CO is putting into Onepass for next year you should be happier in 2004.