Sometimes dishwasher detergent gets stuck inside of the dispenser. This typically happens if the dispenser lid is not shut the entire way, causing moisture to harden the detergent. You get stuck with a brick of solid detergent that does not clean the dishes, and the brick must be removed. Keep in mind you must run an additional wash cycle as the soap did not clean the dishes the first time.
Open the dispenser lid and gently pry up the hard chunk of detergent with a butter knife or spoon. If the entire block does not come out in one piece, break it up into chunks.
Gather the detergent with a paper towel and discard it. Since the detergent has already hardened, there is no use trying to re-use it.
Pour fresh detergent into the soap dispenser, close the lid and run another wash cycle.