Any ideas ?

If you’re using any kind of synthetic nutrients, then yes, you should be checking runoff. You won’t always have to, especially when plants look healthy, but if you have some deficiencies, checking your runoff will let you know if the pH is in a good range or if you have a lockout. If pH is good, you take care of the deficiency. If pH is out of wack, you need to flush and get your pH back into a range where the plant can uptake nutrients. I do Organics so I haven’t checked any of this in a while but for coco, I believe you want your pH to be somewhere between 5.5-6.5