My Ds got really constipated on the S26 AR. We added an extra 5-10mls to what was on the tin which still didn't help.
My MCHN told us it can take up to half to a full tin for their bodies to get used to it. We persevered but went back to just the normal S26 formula as did another lady in my mother's group. After a full tin of the normal one he was fine. You may find after the Zantac kicks in, can take up to 2 wks, things will get easier.

Good Luck, hope things settle down soon.