Your Rapid MFR should run @ 27ftlbs factory set in .22, if you have the .20 @ 24ftlbs factory set, .177 @ 19.8ftlbs & .25 @ 28.5ftlbs, depending on your choice of pellet & the power curve on your fill pressure plus ambient air temp you can + or - 2ftlbs. Without seeing the rifle the main cause for loss of power is weak hammer strike on the main valve, or port obstruction, find a good Gunsmith as your MFR will run happily at +30ftlbs on heavy pellets like Dae Sung @ 28 grain or Eley Magnum @ 30 grain, they are very easy to adjust power up or down on Theobens.
foxon.
Power curve?