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Might be just the switch that selects hammer action that is at fault.
Have you topped up the grease as well? I find when new you have to top
up the grease the most.

I'd take it apart and see what's up.

Out of interest, does anyone know how the hammerstop works on these
little pneumatic drills?

--
Skipweasel
Pay no attention to the man behind the curtain.