A pet accident on upholstery is not really a stain — it is a volume of liquid that has gone somewhere. It lands on the fabric, spreads across the weave, and then keeps going: through the backing cloth, into the foam, and on a cushion often right through to the underside. What you can see on the surface is the smallest part of it, and it is frequently in a different place to where the bulk of it settled, because liquid follows the shape of the cushion once it is inside. This is the single fact that explains everything else about the job, including why so many attempts fail.
The smell itself comes from what the urine leaves behind after the water has evaporated: uric acid crystals and the bacteria that feed on the residue. Those crystals are not water-soluble, which is why plain water and general detergent do nothing lasting to them — the moisture wets them, the smell lifts briefly, and then they dry back out. And they are hygroscopic, meaning they draw moisture from the air. That is exactly why the smell returns on the first warm, humid day: the crystals are rehydrating and releasing the odour again. It is not the accident coming back. It was never gone.
Which is why the work has to be enzymatic and it has to go to depth. Enzymes digest the residue rather than diluting it, so there is nothing left to rehydrate later, and they need contact with the material and enough dwell time to actually work — a light spray on the surface will not reach foam three centimetres down. So the treatment is applied to the depth the accident reached, given time, and then extracted properly. Where a cushion has taken repeated accidents over years, the foam can be saturated beyond what any treatment will recover, and the honest answer is a new insert rather than a third clean. We will tell you which situation you are in at the inspection, and that assessment costs you nothing.