The reason this is an issue is because CO handles this differently than other carriers.

As sbm12 points out, the upgrade inventory has to be there when the EUA runs against the flight. Most other airlines will waitlist you for an upgrade when you're within your window. If a seat becomes available, it gets allocated to the next one on the list.

This nuance is reflected in CO's marketing language:

CO: "Upgrades are processed nightly and confirmed in priority order using the following schedule"