I also have a 81 Shovel (FXS also) with a stock kick arm. Personally, I searched high and low to find a NOS stock replacement because I never liked any of the aftermarket arms I tried. At the time I ran an aftermarket shop and got to try them all out.
The stock kick arm has a ball stop that locks it into place. The back side stop for the ball wears out and the kick arm starts to sag down lower and angles your foot causing the problem you described. Red arrow in pic points to where you should find the worn spot.
The problem with most of the aftermarket ones is they don't fold in and can be in the way of your leg. The stock one tucks in nicely. You can buy stock style aftermarket ones that are a little differant and still fold in.