Sounds like most of your force is in line with the planes movement. You should have your hand on the side of the plane (now the top) and some of your force should be pushing the plane down into the bed of the shooting board to keep the plane from tipping into the work. Another problem is that the blade is not square to the side. the sole being square to the side does not guarantee the blade will be. you may want to check that and adjust as needed.
