If it is a potential problem with pellet skirt/seating, you may want to ask the gunsmith what pellets were used to test fire it before they returned it to you....which ones pass their test.

Skirt diameters vary, I remember measuring up Geco and Hobby in the past, I put the numbers on here somewhere; think Geco were slightly smaller. A smaller pellet might not necessarily help if the cartridges vary too (e.g. too loose fit). Zippy seems to have found the answer with a pellet seater.