I have recently bought a house & just like you had a dishlex 100 in it and found when we used the dishwasher the detergent block remained in the holder after use.
I found that the grey seal under the detergent holder door was upside down, smooth side of seal showing, so I removed seal with a screwdriver (without breaking the seal) and put it back with the ridge side facing out, ie turn seal upside down.
Geoff
April 2008