It's been our experience, here in the hard water areas of the western U.S., that the high heat at the end of the washing cycle to dry the dishes, causes the grains of minerals to solidify. This doesn't happen all the time, but apparently the hardness of the water fluctuates, according to a friend that deals with such things...
Is it a cheap make? Mine is and does exactly that, well it did do until my daughter squirted washing up liquid in it. Now it just suds up all the time and doesn't turn off. In the trouble shooting section of the manual it says Problem: Lots of suds, Answer:Have you put washing up liquid in? Then doesn't say anything on what to do if that's the problem!!