When we moved to Texas, and into a rental home, the Landlord told us though the carpets had been "professionally cleaned" we would just have to live with the giant, brown stains in the upstairs bedrooms because they were not coming out and the Owners weren't prepared to replace the carpet.
I bought Folex carpet cleaner (comes in a spray bottle, plastic, non-aerosol) and it took out ALL the stains, even the ones that looked really old.
You spray it on, rub with your fingers, then pat and press around with a clean cloth...this stuff is amazing! I would recommend you try it!