Why does the spark of my zippo's lessen in time? How can i increase it again?

I have gone through several zippo's already. I use all zippo replacement parts and fuel. I just noticed that after about 6 monts of use, even with new flints, the sparks are significantly less than when it was new. It gets to a point that its really difficult to get a spark. All my old zippo's have a comparatively dimmer spark than a brand new one. First i thought it might be finger oil that aculates on the wheel, but i've tried washing it down with diesel and brushing it. I've also tried lengthening the spring to increase the pressure between the flint and the wheel. I dont think that the wheels have gone blunt because they still feel as rough as the new ones. I've ran out of ideas and now i'm back to disposable lighters. Its sad. Help me