arm moving

Hi. I have owned a Kitchenaid Artisan Stand Mixer with a twisting lock bowl for about 3 years. I have used it successfully for soft doughs such as cookies, and general mixing purposes.

Since I began making bread about 7 months ago I have tried to use the dough hook for several different recipes of several different hydration levels, and every time I have the same problem - once the dough starts to come together it the adjustable arm (the one the dough hook is inserted into) begins to float wildly about, even though I have the switch in the locked position.