I have the same ball-head and have had much the same problem. Make sure that you are really locking the head tightly with the larger wheel. Properly tightened, the head should not move at all.
However, in use I find that there is always some 'sag' between setting the position and locking the head. However, I think that this is partly due to a less than rigid tripod mount as much as the head itself. It is mostly a problem when shooting macro, where I end up trying to guess the amount of sag when tightening.
One thing that can help is using a tripod ring on the lens. However, this only works if the lens can use a ring, and again only if the framing means that the centre of gravity of the camera is more or less directly above the centre of the tripod.