I’m glad you found it helpful!
The cover-sealing issue is a common one I had when I first started out. I also have it sometimes now, as sometimes I get a slightly off/bad batch of parts now and then.
My best advice is to make sure you’re pressing the red disc in as flat and flush as possible. Some of my friends use a sledgehammer. Look at the back, and make sure you can see the edges of the paper and plastic all around the button face. (See steps 20 and 21 of the guide.)