DIAGNOSIS The clamping mechanism on the handle is worn or the serrated clamping ring is packed with debris. It's a common wear item.
FIX Unscrew the handle completely off the tool. Clean the metal band and the serrated teeth on both the handle clamp and the tool's groove with a wire brush. If the teeth are rounded off, replace the auxiliary handle. The part number is often 1 607 001 008.
IF THAT DIDN'T WORK The threaded rod inside the handle that tightens the clamp might be stripped. You can sometimes replace just that rod, but it's usually easier to get a new handle.
HEADS UP Do not overtighten the handle with a wrench. It should be hand-tight only. Overtightening strips the plastic threads instantly.