So your steel might be "worn out" and flexed too many times. You can measure your steel leaves and have new ones made. This is not likely. Although you've probably lost some of the arch over time and you might want to get them re-arched. Get an extra 1" or so of arch if you are going for a NAPCO. You can also have an extra leaf added, which is what I did.
The most likely problem is that you've got rust and road grit between your springs and they have worn grooves into one another. Now they stop acting like leaves which slide past one another to one giant chunck of steel that has little give.
Don't add grease to them to help them slide, as the grease will only catch dirt, dry out and make the problem worse.
Take all your springs apart and grind away that grit, rust, and grooves to make a nice smooth surface. Be carefull with the grinder and keep a light touch. You don't want it to get hot and loose it's "spring temper". Some people will claim that you should keep the springs aligned in the same direction as they were originally, so if you are concerned identify the fronts and backs and left side and right side.
Now that they are all nice and smooth, paing them with some good paint to keep them from rusting rapidly again. Now put some polymer liner between the leaves. I forgot where I got mine, but it is standard stuff now days. I had to trim it to fit for width as my rear leafs are 2 1/2" wide, but you might be able to find some that fit your width exactly.
The polymer liner acts like a permanent lubricant between the leaves and allows them to slide past one another. I have seen a noticable difference on mine. When I jump up and down on the bead, the springs actually spring. It didn't do this before.
Good luck.
