Barbara, I'm so sorry to hear that Tigris is sick again. I know that this is a terrible disease because a lot of the time no one knows why the cats get sick.
Storm's tests were inconclusive the first time and he had a bad bacterial infection the second time and also scar tissue. After his surgery the surgeon said that his urethra was very small so it was only a matter of time before he had problems. I'm still feeding all of my furkids the special prescription food just to be on the safe side. So far so good.
I wonder if your vet can either x-ray or do an ultrasound on Tigris's urethra to find out if it's normal in size. Prayers and positive thoughts are being sent your way. Please take care. {{{HUGS}}}