A lot of stress for people is the sudden rush of the water. I do not know if this is the case for you, but it happens to others. The water hitting their face shocks them, and they are unable to complete the skill. Before you descend, breath off your reulaotr with no maks, and your face in the water. If the water is chilly, this could take a minute before you can breath comfortably. You might even want to hold your nose for the first ten seconds or so, and then let it go. Do this every dive. This will get your face used to the water, and you will not have the shock of the colder water hitting your face when you flood your mask.
Also, I noticed that a lot of people say to take sharp breaths when doing this. I can not understand that one. I guess it works for some people, but for others, slow easy breaths work the best. A large breath can tend to puch the seal to far away from the face, and cause excess water to come back in the mask from the sides. While the sharp quick breath may work for some, it does not work at all for me. With a nice slow breath, I can clear my mask multiple times on one breath.