try doing a search with just the iah-uio-iah flights. when you search co.com it will look for the easiest connections. so if your iah-oak/sfo flights that make most sense are unavailable, the entire trip will show as unavailable.

if you find that your iah-uio-iah flights are good, call CO and tell them your sitch. they will search for other oak and sfo connections for you. you may have to hang in the airport for a while, but it works many times for many CO FF'ers.