Slipage in 1-2nd gear after tranny was rebuilt

I recently had my transmission rebuilt because I had an issue of the truck makeing a jolting type feel in the transmission when accelerating from 1-2nd. It did happened ofthen but eventually it got to be almost a daily occurance. So I took it into a tranny shop and had the tranny rebuilt because the tech said that is what need to be done, well not 2 days after I received the truck back the issue happened again and I brought it back to the guy and I was told I need to drive the truck and left some valve wear down but the issue has been going on for 6months now and im getting fed up with this tech. What should I have this guy do to fix this issue.