Are you using a single point delivery system(one hose)? if so is it possible that you are getting a channeling effect, in that the water isn't wetting the whole root system.if that is the case add a sprinkler to the end of the feed line(s)

I ask because I just went thru something that looked similar,and the remedy was shorting the off time from 90 min to 20 min.

been at this awhile now and there is still so much to learn
Daddy

"war on drugs is curtural genocide"