OK, I should've asked this before really
Is the dispenser lid open or closed when the cycle has finshed ?
If it's open then it's working electrically and something is either stopping the lid opening completely (a dishwasher freshener hung on the upper basket ?) or the tablets are poor and "sticking" inside the dispenser.
If it's not opening at all then there's an electrical fault which will either be the dispenser normally or possibly the PCB control unit, this may need checking by an engineer to be honest as it's impossible to tell without the correct test equipment. Check
Repairs@ if you should need one
Dave.