Indesit DV62WHUK Dispenser?

We've got an indesit DV62WHUK and the dispenser door doesn't open during the wash cycle. The dispenser door operates great at other times (it's got a spring loaded door which works with a little release bar at its base) so it's no problem putting in the detergent but for some reason it will not open during the wash. Also there appears to be no way to access the rear of the dispenser to see what might be causing the problem.

What can I do to repair this annoying problem?