A calcium or magnesium deficiency could cause this. You could test for water for GH (General hardness). The GH test detects both calcium and magnesium. If your water has 3 degrees of hardness or less GH could be the problem. Not all root tab products have calcium or magnesium. Most fertilizer companies under estimate how much of these two nutrients are needed. So many that have calcium and magnesium just don't have enough. Additionally in many places tap water has substantial amounts. So some manufactures of fertilizers don't even add any calcium or magnesium.
Also it is possible for your water to have mostly calcium or mostly magnesium. If so the GH test may not indicate an issue. Another option to trouble shoot this is to get a GH booster at the store. GH boosters have both calcium and magnesium. If you dose the water to increase the GH by gust one degree that would resolve any calcium or magnesium deficiency if it exists.
