Rick, are you talking about the preload adjuster on the top of your forks? If yes, they don't become non-functional, they just only go in so far. If they are in all the way and you need more preload you need to open up the forks and add longer spacers. To start with you could measure the length of the external preload adjuster (from all in to all out) and then cut a spacer that is that much longer than the current spacer that is in the forks.
If you having to add a lot of preload to get the sag you want it could be a sign that you need heavier springs. If you don't have much sag with only the weight of the bike but the correct rider sag (rider on bike)then the springs are too soft. If you have excessive bike sag and the correct rider sag the springs are too stiff.