If you've underwatered and the rootball is solid like a brick, a surfactant is helpful. I use 1/4 to 1/2 teaspoon of liquid diswashing detergent per gallon of water. (not the anti-bacterial kind...it'll kill the beneficial bacteria in the soil) The dishsoap lowers the surface tension of the water, allowing it to be easily absorbed by the peat. (in the potting soil)

Slowly add a cup or two at a time. Let it soak-in all the way, and add more. Keep up this painstakingly slow process till you start to see runoff. One of these applications is all that's necessary, but two are ok if need be. The trick is to not add the water too fast. Let it soak-in before adding more.

Adjust your watering schedule.