What am i doing wrong?

You are having a potassium deficiency Sometimes the burn can appear pale, bleached or yellow, instead brown These are all signs of a potassium deficiency.

if properly balanced and complete, you should feed every time in coco. the runoff takes care of buildup... an occasional flush is needed if you constantly run into issues (once you have a good rhythm down). It's not like soil. You provide everything. I'm guessing you have to jam on the gas. figuratively... normal strength concentration and in proper ratio at a proper pH. unless you fertilizer says otherwise, go for ~6.0-6.5 to start.

I agree with Shagrath, check your waters PH after you added any nutes & check your your run off as your in coco you have a chance to recover if you can nail what it is. you dont have a diary to look at so ... whats ph of run off? was your coco buffered with cal mag or are you adding to feed at all? what other nutes are you adding & amounts ? with coco it is hard to over water, as the plant grows will need more water so best way i find is too feel & know the weight of the pot just after a feed - then keep feeling weight of pot until feels quite lighter the you know plant hungry.