I'm sorry that happened. I had a male mojave who actually died of an RI a little over a year ago. It's really tricky when dealing with an animal that can get sick so quickly. What likely happened was that his immune system was doing an ok job of fighting it off and then the stress of getting him home and settled in his new environment stressed him out. Stress lowers the immune system. RI symptoms show up. Just keep up your injections and make sure to get him checked by the vet when you're done with them.

My guy held on to an RI from April (first round of injections) to September. In August I started a nebulizer, and repeated injections shortly after. He declined pretty fast in late September.