Ceftazadime (Fortaz) is one of the better abx for RI's, but it doesn't work on all of them. Still, if it's been a week and you're seeing improvement then that's a good sign. IIRC when mine had RI's the vet prescribed Baytril + Fortaz administered every three days for a month, and while I saw some improvement the first week like you did, it did take a good bit of time before I noticed a huge improvement.