check to make sure both of your headphones outputs on the interface work! only 1 on mine works, can't be bothered to have it sent off as I don't really need the second headphone output.
You've probably got something set wrong
headphones 1 is the same as what's going out output 1-2 (meaning it's dependent on the master volume.)
headphones 2 is whatever is going out 3-4 (independent of the volume)
If you put your monitors in 7-8, set the volume control to control 7-8 then you can get 3 separate mixes at once.
its funny that rtfm is an abbreviation round here!
I'm pretty sure it's an abbreviation on the internet period.