I plugged in a couple of random dates (10 Oct-20 Oct) on both United and CO websites, and plenty of availability popped up on each. The flights seemed to be the same- YVR-ORD-IAH-GUA and back (the ORD segment seemed to be bookable through DEN, YYC or LAX as well, but all of them fed into the CO IAH-GUA flight. I find CO easier to navigate to find a range of dates, but UAL seemed to have more options on specific dates. Good luck!