I don't know much about washing machines but not knowing about something has never stopped me sounding off about it...
The door will be told to lock by the control board - the control board will need to have two conditions satisfied before the locking mechanism is activated - First there should be a request to run a programme and secondly the door should be closed - both must be true.
If the door is not locking one of these will be true...
No programme selected
Door not closed
Door switch defective & failing to detect door closed
Control board defective - either ignoring inputs or not sending output to lock door
Door locking mechanism defective
It will certainly be nothing to do with the fuse in the plug - changing it shows your level of desperation...
My guess is the switch that detects the door being closed - but it's just a blind guess...
Good luck...
Peccavi
October 2009