This is caused by the combined height of powder, wad column, and shot being shorter than required for a good crimp. When all is in the shell ready to be crimped, the shot should be 7/16” below the end of the open hull.
This can be corrected by:
Using a bulkier powder.
Using a taller wad column.
As a last resort, a small wad of paper can be placed on top of the wad column, at the bottom of the shot, to get it to the correct height.
