My GE profile dishwasher (circa 2002 model) used to do this. It happened intermittently with no rhyme or reason - I suspect it had to do with how the dishes were configured in the racks. If something slightly blocked the water jets' access to the detergent receptacle, the powder wouldn't get knocked out. Then it would get damp, forming a sticky clump and making it even harder for the water to dislodge it.
I solved the problem by switching to liquid detergent. It's a bit more expensive, but Palmolive Eco Plus works really well and is pretty cost effective from Walmart or even Amazon.