no longer blonde help

Can anyone help? I bleached my hair blonde, then toned the brassy away to a medium ash blonde. My hair grows fast and I found my self wanting to touch up roots every week. I don't want my hair to fall out from constant processing, so I decided to stop being blonde and go back to my medium brown natural color. I bought a box of medium brown hair color (permanant, not semi) and dyed it. After 3 weeks I was blonde again (except the unbleached roots)! Can anyone help me figure out my problem? My hair is soft, a little dry at the ends. I was using garnier fructis triple nutrition shampoo and conditioner. I dyed my hair when it was clean and dry, without any leave in conditioner, gels, mousse, hairspray, or anything in my hair. So I bought a second box of dye, 4-6 weeks after the last attempt at going back to medium brown. This time I bought a darker color, thinking that if it washed out it might still end up being darker than it was. Shortly after the re-dye, I switched shampoo to aveeno nourish + moisturize (hoping to prevent my hair from drying due to dying) And still, as little as 2 weeks I was blonde again. Now, 3-4 weeks later, I'm even blonder. What is going on? Does the nutrition shampoo/conditioner coat my hair so that the dye doesn't soak in right? Do I need a clarifying shampoo before I dye dark to strip any coating off my hair? Or does this shampoo soak into my hair and push the dye out where I just need to use shampoo for normal hair instead of moisturizing? I plan to try clarifying shampoo and then something like clairol hydrience with six weeks of color lock treatment.
Any other ideas? I don't want to cut my hair.   I don't want expensive salon treatments, because I know they always say their dyes might just make it worse because I never should have used over the counter crap in the first place and cheap dyes might react with their products.