i just think you need to water more often, and use more water when you do it, if they're in 5 gallon pots i'd think they'd take at least 1.5 gal each every other day, and make sure you're getting a little run through each time you water, the salts from your fertilizer will build up in the pot if you don't get at least 15-20% run through, and will burn the hell out of your roots.
i'm in 3 gallon smart pots and i put at least .5 each through them every day, but i'm in coco, not soil.