It does seem that those that can get 24 in have let the spring settle and sink.
if the tubes are the same and the follower is the same height, the spring is the only thing it can be
have you ever pulled the guts out and put the pad back on and put 24 in the tube and see how much room is at the top.
it will let you know if its even possible and if so how much room the follower and spring need to fit into.
just something to try for info purposes