Are you in soil?...looks like it. If so what is your mix or is store soil (FFOF, Roots...etc) Good soil PH is 6.5-6.8 so you seem to be in range...in soil Phos gets locked out between 4.0-5.5, best absorbed at 6.0-7.5 (although above 7 is way too high) For some odd reason I think that a good flush at 3x will correct it, if everything else is in line I really can't see it being anything other than salt build up and nute lockout as some others have said.
If it is Phos (which I really don't think it is) here are some organic products that could help...if it isn't phos and you add these you could have a few more issues so a good flush would probably be the best first step.
Organic
Dr. Hornby's Iguana Juice Bloom (4-3-6)
Advanced Nutrients Mother Earth Bloom (.5-1.5-2)
Fox Farm Big Bloom (.01-.3-.7)
Earth Juice Bloom (0-3-1)
Pure Blend Bloom (2.5-2-5)
Pure Blend Pro Bloom (2.5-2-5)
Buddswell (0-7-0)
Sea Island Jamaican Bat Guano (1-10-0)
Indonesian Bat Guano (0-13-0)
Rainbow Mix Bloom (1-9-2)
Earth Juice Bloom (0-3-1)
BIO BLOOM (2-6-3.5)
AGE OLD BLOOM (5-10-5)
ALASKA MORBLOOM (0-10-10)
METANATURALS ORGANIC BLOOM (1-5-5)