This is no news and one of the annoyances with the Elizabeth. It is related to many things: Coffee, dose, pressure, etc.
I get them most of the time. The key is to remove the portafilter and wiggle a bit so the puck dislodges.
Friend of mine with an ECM Synchronika got them 25% of the time. So go figure.
I actually went on and tested this almost to exhaustion, even took it to Bella Barista: Using a different grinder (Eureka 83mm flat) I pulled shots for 2 hours and had no sticky pucks. Went home using the Niche or a Feldgrind and guess what? Got stuck pucks!
I can confirm that using some IMS baskets solves the problem for me at least. I currently use a VST 18g and accept the stuck pucks. Or use a puck screen. :-)