There are only a few things that go wrong with this particular skill. Many divers just get it (myself included), and for them, it is really difficult to figure out what to say to someone who is having trouble. I managed to learn a couple of things when helping my wife.
1. If the bubbles are coming out your regulator when you are trying to clear your mask, you need to learn to breath out your nose. This comes more easily if you spend some time swimming under water without plugging your nose. Eventually, you will naturally breath out your nose to keep the water from coming in.
2. You don't have to clear your entire mask in one breath. My wife was blowing so hard to clear her mask that she instinctively breathed back in through her nose when she was done. If the mask wasn't completely clear, she choked. The trick is to clear it in several smaller breaths. Take your time. A flooded mask is not an emergency situation, and you don't have to rectify it quickly. As my wife is a nose breather, when she started taking her time with the mask doff/don, she would have her mask clear before she officially started clearing it.