Are you sure you're eating over 5000 calories per day or did you just throw that number out there because you "eat a lot"? You can total up your daily food intake at www.fitday.com they have a huge database of foods with protein/fat/carb contents.

If you can't gain weight on gear, there is a problem and it's most likely your diet. Re-evaluate your diet and training and post them in the appropriate forums for critique.