Hi there - mine can do that - you just need to make sure that the flappy bit that creates the block is on the outside of the lip of the seal so that it cant be sucked back in. If it is on the inside then it can be sucked fully open and air rushes in. Hope this makes sense.
yes thanks that does make sense - i did try prodding and poking the flappy bit but to no avail. I didn't want to mess with it too much in case I broke it and then wouldn't be able to return the mat...
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