Go with your instincts... if you think it's the Step2 formula, try changing back for a bit...surely it won't hurt to try it & see?

I know it was a while back (ok, almost 15 years ago) but I fed my DD1 & DS1 on S26 and there was no step2 (that i knew of, anyway) - they just had the same thing right through to 12 months.
So I am thinking it couldn't possibly hurt to change back for a couple of weeks at least, just to see?