I found that i was getting Pre-check when I booked travel directly on the Conited website, but not when using my corporate (Carlson Wagonlit) system. I discovered that on UA's system, I just had my middle initial plus a period. On Carlson, I just had the middle initial with no period. So, I just edited both to add in my middle name - which wasn't an option many moons ago when I first registered on United - and since then, I've had Pre-check at IAD 100% of the time.