I know many talk about immediate relief from the prednisone, but it took me about 2 weeks to get the full benefit, so you may improve over the next week or so. I recall reporting to my doctor that there had been a disappointing 70% improvement only in the first couple of weeks.

You don't drop to 12.5mg and sit there for a few weeks? The jump from 15mg straight down to 10mg is quite a lot. I had 4 weeks on 12.5mg before reducing to 10mg, with drops of 1.5mg then 1mg at a time, down to 10. The general rule seems to be not to decrease by more than 10% of the current dose at a time.