Examine your pieces first to see if:
The steel shot is hammering itself on to the parts and leaving small
dents, your pieces banging into each other and leaving small dents
or your castings have porosity which is collapsing under the
burnishing action of the shot.
If it’s porosity, it’s a separate issue.
If it’s caused by the steel shot, slow the tumbler down and make
sure you have enough liquid.
If it’s the pieces banging, slow the tumbler down, remove some
pieces and make sure you have enough liquid. Too small of a tumbler
can present problems if the pieces are large as well. The burnishing
action takes place as the pieces and shot slide down the front of
the rotating mass, if the pieces are too large for the tumbler, the
don’t really slide, they get tossed like clothes in a spin dryer and
go bang, bang, bang.