The only ones I've had problems with are the 24mm case. It is fairly common to very carefully peel off the outer layer of paper. Use your fingernail and grab a corner. Then peel of the outside layer. They'll go right in after that.
Sometimes the delay liner is too tight also and I have to peel off a layer there.
The 24mm liners are sometimes slightly too large to fit. For those I peel off the very outer layer - only the glasine layer not a full layer of paper. If yuo do this make sure the liner really will not fit before peeling and remove the bare minimum from the liner to get it to fit. Mine are still a very snug fit after peeling.
Most importantly I believe that peeling the overly tight liners only applies to the 24mm motor. All other motors the liner should be a good fit straight from the packet.
Yeah, the 24/40 is fairly notorious for that. The 29/40-120 doesn't seem to have this problem, but sometimes I'll have to peel the outer layer of the spacer tubes from time to time. No biggie. Have fun!
The problem is something that Aerotech can not really control (any manufacturer can have this problem).
When they order cases, and liners, they are garunteed to be + or - a certain amount in spec. What happens when they order cases and liners that are in spec, but the cases are on the - side, and the liners are on the + side. They won't fit. And since they order them in lots of thousands... they have to use them because they are within spec, and they cost to much to replace.