As a newbie to this forum and as I had this exact problem, here's my input.
Having read all the posts here and other places, I decided to check out the infuser piston as a likely area. Using the very good vdieos on youtube to explain how to strip and clean the infuser, I did this. A word of warning. Use only silicon grease. I initially tried lithium as I didnt have any silicon grease around. Had to do it all again as this was only made matters worse.
So I stripped the piston out, as per the videos, cleaned the o rings and channels, which after two years were real mucky. So using the silicon grease, which I had to buy in a large spray can as I couldnt find any of the small tubes they used to sell, sprayed all the moving parts and the o rings, and gently replaced the o rings and piston.Then found out there are two ways to insert the piston back into the infuser, the right way, and the wrong way. Wrong way takes about ten minutes to work out why it doesnt all fit.
All back together and for the last two weeks coffee as good as the day we bought it, and minimal spillage of grounds inside. The discarded pucks are solid.
So thanks to all the previous postings, and really this all takes about half an hour or so to do. Tools needed, small screwdriver, dry cloth, silicon grease, and common sense when replacing the parts. The o rings are fairly tough, but dont push your luck be gentle with them.