The stuff that is not dissolving in Barr's GH booster is Ca SO4 <The other stuff is Mg SO4 & K2 SO4, both soluble.>
I have not use Barr's booster, so can't speak of it first hand, but I would expect it to behave like Ca CO3. It takes a while for that to dissolve ... and there is an upper limit to how much can dissolve in.
I would try letting it sit for 48 hrs & test your GH to see if anything changes. The amount that can dissolve in is pH dependent, but if you use distill water, you should see a GH increase.
BTW, instead of adding stuff to your softened water, why not just mix in some water before the softener?