Last summer I installed msd coils, bbk 75mm tb, bbk cold air kit, CnL upper plenum and a predator programmer. the car already had an american thunder exhaust and an off road x-pipe. On the dyno the car had 250 rwhp 287 rwtq.
This year I had installed BBK ceramic coated shorty headers, spec stage 1 clutch, spec alum. flywheel and 3.73 gears. after the required break in for the clutch and gears it went back on the same dyno. 231 rwhp 267 rwtq. I reflashed the diablosport tuner, same grade fresh fuel , clean Mobil 1 oil. I was hoping for a gain of 6, but lost 19. any suggestions? I am at a loss?. Oh, and only 1500 miles difference between dyno runs.  

It's probably the difference in the gear ratio that has changed your readings. The torque multiplication is different from gear to gear. We've seen this happen a number of times before. the better gear ratio will more than make up for the loss in HP and torque.