Is it underwatering now ?

Overwatering coco isnt possible, but over concentrating fertilizers and chasing down a PH certainly could be reason for what you are seeing. The concept of "fertigation" is very flawed that you read about online, it creates a buildup of fertilizers/salts which will cause your PH to be all over the place throughout the areas of the root zone, and not to mention a waste of nutrients. @Roberts is 100% bang on for coco, like @Roberts said, maintaining that digital PH at 5.8 (or a yellow 6 to us that use chemical testers), is how I grew in coco pretty successfully too. @Roberts helped me grow a nice plant. In coco I kept the substrate irrigated the entire grow, but would alternate between nutrient feeding and just 6.0-6.5 (i use chemicals not digital PH meter) to allow for the nutrients to make it thorough my substrate and into runoff. Flowering was always PH 6.5 yellow/green on the tester, grew awesome flowers. Also, I noticed that the coco I bought had an incredibly high PH when I tested soaking water, so high my chemical tester turned purple... so higher than 8.0. It took me about 4 days of buffering my coco coir with PH 6 (yellow) water before my runoff PH was consistent. Good Luck!