have you been getting any run off when you water? has the pot been absorbing it all, with vert little leakage out the bottom? all i can really think of at this point is bad salt build up in your roots. if thats not it, i'm stumped.
at this point i'd water it with plain, ph'ed water till you get a fair amount of run off, then give them a light feeding with a good dose of cal-mag, ph'ed to 6.1. i think i'd let it dry out and repeat that cycle a couple of times and see if it helps, then resume regular feeding, remembering to get a little run off at least every other time