Feeding right after repotting (coco coir).

Coco is very ph sensitive. You need to get a run off reading of your ph. If it is out of the 5.8 to 6.2 safe area them that is the issue. It needs to be flushed to correct ph then re fed a fresh solution mix. Would be first thing to check. Also sounds like you have a salt build up from not enough irrigation. Coco is a high irrigation type of grow. It is Hydro in a potter and should never dry out.

Dr Bruce Bugbee did a good video explaining what he does in his labs. Say they mix a 1EC nutrient solution and pH of 6.2. They ALWAYS have about 10-20% run off in which they test that water as well. He stated that if your media is well balanced, if you put 1EC in, you should get 1EC out. Likewise if you put 1EC in, and you get .5EC out, then your plant is consuming more then what you are giving it and should increase the amount till you reach a point where it stays at your target range. If you put 1EC in, and 1.5EC comes out, then you had a buildup in your soil. I’ve been following that practice to the “T” and I always make sure I get around 20% run off. I have yet to have a different EC reading on my IN vs OUT. I suggest you mix up a batch of higher PPM solution (my honest opinion is to only measure EC though) and make up enough to where you will have roughly 20-40% run off. This will get you back on track. Hope this helps.