IMO in pretty much every single RPG... Fighter types are only as good as the sword or axe they are swinging and the armor they are wearing, they are gear dependent... while casters can run around naked and still do pretty good. How to fix, just gear your fighters better. Im sure the casters in your group will be glad to know you gave those fighters awesome gears to boost there effectiveness. If there jelous... well then there just selfish and probably jerks. As long as people are having fun. Magic is supposed to be powerful.... its magic!