Why won't my DVG 622 soap flap open during the cycle?

I have an Indesit DVG 622 washing machine, which was bought new approx 3 years ago.

Over the last few months I have had to run a single load of dishes 2 or 3 times because the soap flap doesn't open. I am not operating the machine any other way than I have in the past. I am using soap, rinse and salt recommended by the manufacturer.

Any ideas what I can do to correct this problem?