Neff dishwasher - detergent dispenser won't stay shut!?

Hi
I have a new Neff dishwasher.
Whenever I use any cycle other than the full 2 hour cycle, the detergent dispenser pops open seconds after I've turned the machine on. This means that the tablet drops to the bottom of the machine and the cycle continues without any cleaner. Does anyone else have the same problem? And know what's going wrong? I've tried opening the door and replacing the tablet but the lid just keeps popping open!
Thanks
Sarah