There is a possibility that the brake fluid has broken down some or has gotten some water in it. That can make the brakes feel spongy too. Since you've already changed the brakes and bled them, I would take a look at the master cylinder and the brake lines. If there is not enough fluid, that can cause a problem too. Really, since brakes are failry simple, it can only be one of a few things....brake components (rotors/pads/drums/shoes), brake lines (leaks/crimping), master cylinder, brake fluid. Systematically check each to make sure they appear in good working order. If they do, and the problem persists, then the only thing left is to take it to a mechanic. Typically, dealers are way over priced, and a decent/reputable brake shop would be a much more economical way to solve your problem.
Good luck!