how can I fix the tablet dispenser on a diplomat dish washer?

I have a diplomat dishwasher, only just over a year old, from MFI Can't remember the number but similar to those mentioned in other questions. I use it about once a week, last week it did not drop the washing tablet at its usual predetermined time.
After the cycle completed I opened the door to find the tablet still there if a little smaller due to the heat.
The flap had not opened.
I am loathe to call out an engineer for what should be a simple problem.
I have of course since tried all the programs with the same result, the tablet compartment does not open.

Dave
July 2006