I had this exact same problem when I was trying to learn to clear a mask, and it was due to reading the instructions and following them precisely.
The problem is one of anatomy. When you look at a person's face, it looks as though their nostrils go upward along the length of the nose, but it isn't so. The air passages actually go straight back into the face, and then curve down and intersect the mouth and throat. If you are kneeling in the water with no mask on AND you are not breathing out through your nose, it is easy for water to run along the base of the nasopharynx and down into your throat. If you tilt your head back from a normal, upright kneeling position, it makes it even easier.
The head tilt instruction is designed for a person in a normal DIVING position, i.e. close to horizontal. In that position, if you are looking down toward the bottom, the lowest point in your mask may be the lenses. In order to get the bottom border of the mask to be the lowest point (where the water will go when you fill the mask with air) you have to tilt your head back. It is NOT at all necessary to do this when kneeling!
A couple of other points: The volume of air it takes to empty a mask of water is FAR less than the air you take in in a single, normal breath. You don't have to use a whole lungful to clear that mask! You also don't have to blow HARD; it's the air that does the work, not the force. You can trickle air into the mask, and it will accomplish the same thing. (These points are even more important when you have to do this while actually diving; the instinct is to take a big, deep breath and WHOOSH to clear the mask, but this actually screws up your buoyancy, which is a bigger problem than water in the mask!)