I have never used cardboard for shims, but what works for me is simply taking a piece of scotch tape and making one trip around the top of the insert then puting it in the ball. This will definately give you that little extra.
Another thing to consider is your ball driller could have gotten a new bit for the fingers. Since that bit is used the most often of all the bits, it wears down a little faster and needs replacing. Your past few balls that use the same inserts may have had a smaller, worn down, bit and now with the newer, bigger bit, the inserts seem big.
