A similar thing happened to me.

Was initially on 20 mg/day of prednisone due to PMR/GCA. This was good at first, but then the pain started creeping back over a few weeks. What happens is that your body senses the prednisone, which acts as synthetic cortisol, and so slowly stops making its own cortisol. This takes a few weeks, during which your pain will increase. In my case, after about a month, the inflammation was so severe that ER doctors then put me on 60 mg/day of prednisone due to an episode of double vision.

I recommend getting CRP and SED inflammation scores. If they are still high, increase the prednisone incrementally until the CRP and SED scores come down to near normal. Then you've found the level of prednisone you need. For me 60 mg/day was more than enough; I actually think 40 mg/day probably would have been enough.

In my case, I also started on weekly injections Actemra after about 5 weeks of 60 mg/day of prednisone. I then tapered off the prednisone over a period of 6 months while continuing the Actemra. I ended up staying on Actemra for 2 years. It was very effective for me and I suffered virtually no side effects.