Sorry if these are dumb questions..

Are the pins that transfer wheel cylinder effort to push the shoes out completely engaged in the shoes?

Were the bosses on the backing plate grooved or worn excessively?

Was the brake adjusted after your rebuild? Drum spins, light friction without brakes applied?

Adjuster facing the right way?

Do both shoes show evidence of touching the drum?

Does one shoe (primary or secondary) show more evidence of touching the drum? Might look closely at the top edge of friction material - closest to the anchor boss, or at the bottom, closest to the adjuster.

If one shoe is showing more evidence of wear than the other, it might lead you to what is happening.