Sounds like salt build up.. you have hot spots in your soil that need to be completely flushed from the sounds of it.. How do you water? Small amounts daily? Or drench the pots and leave them for a week? also.. how often do you feed straight water? Are you growing in pots or beds? If its in pots.. rinse with PH 6 water allowing generous run off then DONT FEED for at least 4-5 days.. you want to try to loosen the salts in the soil and allow the plant to use up the nutes that are already available before adding anymore. Next.. follow a schedule of 2 feeds 1 water, and give them a little bit more of a soaking when you give them water every third day. This will ensure that excess salts will not build up in your soil. If your growing in beds.. feed small amounts of water daily untill your ppm's are consistantly back in check (should only take 4-5 days at most).. then continue with 2 feeds 1 water. Hope this helps! Cheers!