If I remember correctly, HaT's figures are of a soft plastic, rather than styrene, like the old (and current?) Airfix or Italeri figures from back in the day.  Paints adhere with difficulty to that plastic.  I ran into that same issue when I was a kid in the 70s and painted Airfix and Atlantic figures.  I used the same enamels and acrylics I used on my models-Testor's, Pactra, Model Master.  And I noticed that with handling, the paint flaked off very easily.

I know I've seen posts with tips on prepping the surface of this softer plastic, though I don't recall if it was here in the Herd or in another forum.  But I know that one old trick was to coat the figures with thinned white glue (eg, Elmer's), aka PVA glue, as a kind of undercoat for the primer.  Then apply the primer and finish colors.  The white glue provided a little bit of adhesion, that resisted the forces on the figure's surface that lead to flaking.

If I can find the references, I'll share them back here.  But I suggest searching on "airfix" and "figures", too.  Maybe you'll find similar tips.

Best regards,

Brad