How I was taught this test is to fit the mask without the strap, do a little inhale and hold (stop inhaling). A mask that fit your face well will just stay put without having to inhale again. These a sure bet for not leaking. Those that fit fairly well will very very slowly start to loosen. With the water pressure, these will usually be good too. Do it with a reg in your mouth too.When I bought the mask, I did the thing where I put it against my face and inhaled and the mask stuck to my face...
As someone mentioned above, the straps do not cause the seal and in fact tightening may cause it to leak. The increasing water pressure outside will keep a properly fitted mask on your face.
Sounds like this mask is not suitable for your face.