When hammer heads are angled on their shanks, they can sometimes require trimming (bevelling) to give clearance. I’ve posted on this previously (Hammer Cutting) but these pics show more clearly how to select the required angle (the amount) of bevelling: Simply, cut the felt in line with the hammer shank –

cut in line with shank

bevell is cut

Sometimes, this brings the cut close to the felt staple, so this too needs cutting/filing (otherwise you’ll get to hear that sickening sound of a beautifully sharp blade striking metal), and the felt perhaps needs rounding off.