Have you read the book "Knife Throwing: A Practical Guide" by Harry K. McEvoy? It has some good pictures of correct form. You might also try some of the books by Ralph Thorn on Combat Knife Throwing. They are available on Amazon.
When you say your knife does not stick, is it because it is going point first but not penetrating the surface? Or is it not sticking because you are hitting with the blade or flat rather than the point?
If you are hitting with the point, but it is not penetrating the surface, it coud be that you are throwing against too hard a target or have too light a knife.
If you are hitting with blade or flat instead of the point...
Do you hit your bullseye or are you missing it? If you are missing, That would be a good indication that you are not releasing at the right time--either too high or too low in your swing.
If you are hitting the bullseye but hitting with the blade rather than the point, try adjusting your distance. Do you release with the foot of your throwing side forward or back? Are you stepping into your throw or throwing from standing? You may have to change the number of steps or change the length of the steps to get the perfect knife rotation and hit with the pont.
If you are hitting with the flat, then you are likely adding some sideways arm movement or body rotation in your throw. This asumes you are throwing overhand and not underhand or sideways. Try carefully to execute a vertical arm swing.