Cleaning with an ammonia based product just makes the problem worse. The ammonia attracts them to the spot for a repeat performance. Use vinegar or a product like Nature's Miracle or Simple Solution. Simple solution makes a formula especially for cat urine & spray.

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It would really help to add at least one more litter box. I have 7 cats and 8 litter boxes in the cat room alone. When I let the cats out at night, I add a litter box near the livingroom. And my house is very small.