Manganese is used in such small amounts that I would suspect a ph lockout or nutrient imbalance preventing its uptake. Or, I'd suspect magnesium just because that occurs so much more often.
I would either
- Flush. The last gallon containing mild nutrients. This would test the theory of lockout/imbalance. It would be a reset.
- Add 1/2 to 1 tsp/gal epsom salt to the next feeding. This would test whether it's mg def. If problem continued, I'd do the flush.