How are your pads? If they're worn down to the backing plate you could be pushing as hard as you can and all you're doing is pushing the steel backing plate against a cast iron rotor, which won't do much of anything.
Its also possible that you have a siezed master cylinder or caliper piston, or a kinked brake line...lots of things could be going on here.
Its also possible that you have a siezed master cylinder or caliper piston, or a kinked brake line...lots of things could be going on here.