soap dispenser does not open?

I have a hoover DH835 dishwasher and since we started using the all in one tablets (including soap and rinse aid) the soap dispenser does not open during the cycle, leaving the dishes dirty.
Any suggestions, we have put salt and rinse aid in the machine since and given it a good clean out, still no change.

What is causing this, what needs to be done? Your help is much appreciated