I can't comment on your problem, but as I also grow in coco and use canna nutes I have noticed a couple of things you need to be aware of.

The PH range for coco is 5.8 to 6.3 (I aim for 5.8/5.9). My local grow shop also recommends that if you are using hydro nutes in coco, feed at 3/4 strength (this has worked fine for me, if a shop tells me to use less of something that they are selling me I am inclined to believe them - they also have a great reputation for good advice).

:hippy: