Hi Have you tried Simple solution? I think you get it from the petshop. Or another specialist pet odour killing product? It has to have some kind of enzyme action to break down the residual odour. You need to absolutely saturate the spot too, because the wee sinks through to the backing and spreads out under the carpet, and you're right, the smell will keep drawing them back to that spot.
Do test a little area first though just to be sure it's ok for your carpet.