2003 SS transmission problem

When taking off, the transmission seems to be hanging up in 2nd gear, I can press the gas, and nothing happens, it eventually will kind've bounce into the next gear. When I took it to the dealer, they found debris in the valve body, and cleaned and rebuilt the valve, but this didn't fix the problem. Can anyone tell me what might be causing this?

I had a similar problem in my 02 SS. Dealer said catalytic convertor was restricting airflow so bad it couldn't shift properly. They replaced, and no problems since.