Happens all the time ....

-reset the nutes...(I change mine every 2 weeks...)
-ph adjust ...

what is the distance from the plant to the light .... with the 400 in a jam setup ... the light decay gets to be to much for the lower leafs to keep going ... perhalps you need some suplimentary lighting ... around the base area..... (if the leaves are that important)

hope that helps