Originally Posted by findark

Hard to say without specific flights/dates, but this situation is usually because of inventory, which would also restrict your ability to SDC.

As jsloan notes, routing rules permitting more than one stop are very uncommon these days, so it would be challenging to change to a two-stop itinerary ahead of time. Once the LGA-IAH coupon is lifted, you should have fewer problems SDCing by app, although rumor has it the app has tightened up a bit recently.