Nature's Miracle works, but there are a few tricks to it. First, it must be fresh. If you have a bottle and keep it in storage for a year, the enzymes die and it will no longer work. Only buy what you need and use it fresh.
Completely soak the area, don't spray it on, pour it. It has to get into the carpet pad. Then let it air dry completely. You can lay foil over the area as it dries. Don't use a steam cleaner until it is 100% dry (about a week). The enzyme eats part of the urine that smells, once it does it's work, the smell will be gone. It will look dirty, so after it dries you can use the steam cleaner.
As a warning, once you pour it on, for the next 24-48 hours will smell much worse then it did before. This means it's working and as it dries the smell will disappear.
Once you figure out which cat is the problem, I'd recommend you go to the vet and make sure there's no urinary track infection. If it's not a UTI, after cleaning the carpet, spray the area twice a day with Feliway spray, that's a pheromone cat scent. If they smell it they think they've already marked the area. The spray works better then the plug ins.
If you still find pee, you can talk to your vet and ask for behavior medicine. I had to put my cat on feline Prozac (fluoxetine) for 6 months and she's been on good behavior for about 2 years now since finishing the medicine. If you do end up going with medicine, I'd recommend paying extra to get it as a liquid. It's easier to give liquid medicine to cats then pills and easier to gradually reduce the dosage to wean them off the meds.