Well when using RO water you are starting from a clean slate so with that in mind that's when using gypsum, calcium chloride, etc comes into play thus you don't have to worry about anything in your water that's going to effect your brew or growth of your yeast, yeast nutrient in the boil and...
Had the same problem, use 1/4 Camden tablet per 5 gallons in mash, also I would recommend scrubbing the heck out of your fermenters! Ever since I used the Camden tabs never had a issue!