PH BURN?? Whats happerning?

Hi peeps i have a prob but let me set the scene first. Strain is BC Chronic from clone. 600w per square meter SOG veged for 3 days after fully rooting in ebb n flow with canna veg nuets with a little flormulex added then switched into flowering 10 days ago the fisrt week they remained on veg nuets then on sat i switched to flowering neuts which are again canna a+b neuts the plants were all looking very healthy but today i noticed some of the leafs on the sliightly larger clones are brown and curling. Now i havnt changed the nuets since sat and the tank was a bit low and PH was 6.11 canna recomend 5.5 to 5.8, ppm was 600ppm but thats perfect. Now this dosnt look like nutrient burn because its starting from the center of the leafe blades along the lines in the leafe blade. Its only on the larger clones as well. also its not light burn cos its just on one table all the others look fine. but each table has a seperate res tank so im thinking it must b ph.. can some one give me some more advice on this.
thanks ziggy