My dog had a few accidents on our couch a couple months ago and even though we cleaned it with enzyme cleaner at the time, the smell keeps coming back, especially when it's humid. We're considering replacing it but wanted to try one more deep clean first.
The smell keeps coming back because enzyme cleaner alone probably didn't reach deep enough into the cushioning where urine soaked through the fabric. Try pulling off cushions and spraying the exposed foam and frame with an enzymatic cleaner, let it sit for 24+ hours, then vacuum or blot thoroughly - this targets where the smell is actually living. If that doesn't work, sprinkling baking soda on damp cushions overnight and vacuuming it up can help neutralize lingering odor. The humidity thing makes sense since urine salts reactivate when moisture returns, so keeping the couch dry and occasionally airing it out will help prevent it from coming back.
Have you tried pulling the couch apart to get at what's actually underneath? The thing is, enzyme cleaners work great on surface stuff, but dog urine wicks down through fabric into the foam padding and wooden frame itself - places no spray can fully saturate. You need to either soak the affected cushion cores in an enzyme solution for hours or consider replacing just the cushions rather than the whole couch. If the frame or wood slats got hit, that's tougher, but a vinegar and baking soda paste worked surprisingly well on the wooden base of mine before I sealed it. The humidity bringing the smell back is telling you there's still moisture trapped in there!
Baking soda left sitting in the cushions for 24-48 hours before vacuuming can pull out lingering moisture and odor that enzyme cleaners miss, especially if you sprinkle it after lightly misting the affected areas so it penetrates deeper. If that doesn't work, your last resort before replacing would be renting an upholstery steam cleaner and running it multiple times over those spots - the heat and extraction combo is way more aggressive than anything you can do by hand.
Replacing might actually be the smarter move here depending on how deep it went - sometimes those accidents create a chain reaction where bacteria keeps regenerating even after cleaning. If you want to give it one last shot though, try white vinegar soaked into the affected areas followed by the baking soda method people mentioned; vinegar neutralizes the uric acid crystals that enzyme cleaners sometimes miss, especially in humid conditions when moisture reactivates them. The combo of vinegar first, then baking soda overnight, then vacuuming usually catches what single treatments don't.
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