Concrete is permeable so what you have is a circumstance where pet dog urine has actually been soaking into the permeable surface area (it seems like continuously) for a couple years. A minimum of a few of the products you have actually used are working.
The enzymes work by absorbing the urine residue. The reason you keep them moist its two-fold.
One: They need to be moist to be active. When they dry they do nothing.
2: To obtain them to soak into the concrete far enough to reach the urine, which would be in rather deep.
Bleach makes use of oxygen to break down the urine. It will work also if it can reach the urine in adequate concentration and for a long adequate time to work (which for oxygen is fast). Cleaning is excellent and it will remove the urine it can get to. You do have to beware with power washing though due to the fact that it can drive the urine even deeper into the concrete depending on exactly how you do it.
Neutralizer you asked about also has to be available in contact with the urine to change the molecules. So your challenge is to obtain one of these (a good item) deep enough into the concrete. Where the urine wants a couple years of taking in. One suggestion is to apply an item and keep it moist for a number of days. The longer it is moist, the much deeper it will permeate. Likewise, ensure that the urine scent is not coming from surrounding items. Exists wood close that might be infected or fabric or even dirt? Urine will wick up a number of inches into other products when present.
It is common not to be able to entirely remove all the odor in a circumstance like yours (e.g., concrete or even wood). So a choice used by specialists is to seal the concrete. After you have actually eliminated most of the odor, you can then seal the concrete with a good pigmented shellac, varnish or acrylic sealant. These sealers have actually tiny holes left in them when they dry out. These holes are larger than the odor molecules so odor can still get through the sealant. If you apply 2 coats of sealant this will help eliminate that concern.