Here is what the American Cancer Society website says:

❝The pattern of the drop in PSA after radiation therapy is also different from after surgery. PSA levels after radiation tend to drop slowly, and they might not reach their lowest level until 2 years or more after treatment.

❝… Some medical groups have proposed that if the PSA rises more than 2 ng/mL above the lowest level reached, further treatment should be considered, but some doctors might advise tests to look for cancer in the body even if the PSA has not yet risen this much.❞

So definitely talk to your medical team about any concerns, but what you're seeing only 9 months after radiation is consistent with what the Cancer Society seems to consider normal (even if some people in the forum had much faster drops; we all respond differently.)
https://www.cancer.org/cancer/types/prostate-cancer/treating/psa-levels-after-treatment.html#after-radiation-therapy