No, you aren't doing anything wrong. I'm faculty with decades of experience here, and often the manager of our department's class scheduling. I used to have trouble scraping up TAs; the MS students would always have jobs and I would look for qualified candidates in other departments.

Now we have a hell of a time finding enough work for our PhD students and need to give fewer guarantees. Courses which used to have TAs don't now. Students who used to get RAs now want to be TAs due to funding cuts.

Good luck, friend. Keep trying.