My thinking is, as suggested by my British Mentor,
not enough power.
I'm kinda 18th century in how I judge shotgun results... I don't chronograph velocities, but use cause-and-effect...and a pattern board
Using 2Fg in my flintlock trade gun with cylinder bore..., all squirrels fired at were harvested.
Using 2Fg in my Pedersoli caplock SxS (has choked barrels) ..., squirrels, partridge, rabbit, and pheasants, all harvested.
Using 1Fg, powder measure set at same load as 2Fg..., nil for three tries on squirrels. This was in my "squirrel woods" where I normally hunt them so my distances are consistent.
I haven't tried the Tuna-Can test, at 25 yards for 1Fg, but my hypothesis is it will be a poor result.
(Empty, steel Tuna can, bottom toward you at 25 yards, shoot your shotgun and see how much if any of the shot penetrates. Either adjust to a bit more powder OR larger shot, if results are poor, and retest)
LD