Our preference, since we are impacting on or near tool edges, is the plastic tool tip. However, when the endmill diameter because smaller and/or longer you may get repeated error messages that say "hit harder". The plastic tip is absorbing too much energy by making the impulse duration too long. Switching to the metal tip will shortens the duration the hammer is in contact with the tool, allowing it to reverse direction more easily. It now needs a lot less force so you should get successful measurements. On the other end of the spectrum, larger diameter indexable milling tools and you get "hit harder" messages, you should add the mass or weight to the hammer. Screw it in to the hammer opposite the tip and change the sensor setup, selecting the hammer with mass.
UPDATE: We have added a new large diameter plastic tip: https://mli.supportbee.io/2770-millmax-support-center/6000-new-features/19417-large-hammer-tip