Are you using an enzymatic cleaner? Theoretically the enzymes will neutralize the urine odor. If there's a pad under the carpet, you have to make sure to use enough enzymatic cleaner to soak the pad, as well. We use Simple Solution and for stubborn odors it suggests soaking the carpet and pad at that spot and allowing it to air dry. When dry, the odor is gone but there's a slight residue that I wipe off the surface with a damp paper towel. It's always worked really well for us, though I can't say that I like the odor of the Simple Solution much better than the urine odor.
At least the SS scent dissipates with time, though.
I know a lot of members have had luck with the homemade recipe and technique luckypenny mentions here:
Originally Posted by luckypenny
When we had carpeting at our old house, I thoroughly shampooed and then rinsed them. Once dried, I'd pre-soak any pee areas with 1/4 cup vinegar, 1/4 cup rubbing alcohol, 1 cup water....and then pass the carpet cleaner again with just vinegar added to the water instead of shampoo.