HCS,
Cat urine (particularly un-neutered toms) is amongst the most persistant of odours. If the spot has been re-visited several times it may require renewal of underlay to effect a cure.
I normaly give a heavy pre-treatment of Prochem B153 Urine Neutraliser, tamped in and with a long dwell. After cleaning a post treatment with Stapo Stakill is left in the carpet.
The above subject to normal pretesting.
John.