Let me see if I can help you on line and save a few pounds.
I note bottom barrel is the weakest strike. This is normal for the following reasons,
(1) it gets the most work in most cases as most shooters prefer to shoot bottom barrel first.
(2) The lower pin gets struck with the middle of the hammer rather than the top. Example is hold a normal hammer half way down the shaft and try to drive a nail in - not as efficient as holding it in the normal position.
Things you can try for yourself are swap the top spring for the lower, by rights it should be fresher.
Do not shim up as this may c