2000 zx2 auto trans question

My car has lost acceleration power and it sputters.Air filter,spark plugs,fuel filter all have been changed,but the cars performance hasnt improved.So I go to have my transmission flushed thinking that would be my next step,but the man tells me the fluid doesnt want to come out,with the exception of two quarts.Any suggestions on what this might be,and where to go from there?