Coffee Biscuit doesn't come out in Knock Box

We have a Breville Dual Boiler Coffee Machine, my husband even made a barista course to learn all about it. But when we try to knock the coffee biscuit into a knock box, it doesn't work. It never comes out and if you use more force, the filter pops out before the biscuit (hope it's clear what I mean). So you need to get a spoon and scrape the coffee out of the filter (and avoid doing a big mess). Does someone knows a trick or has an idea why does it happens. I though, I use to much pressure to compact the coffee at the beginning, but according to my husband it's okay. Thanks for your help Bettina