I had a similar problem on my Rocket R58. There was no whoosh of water exhausting after a shot or after a blind flush. My pucks were wet too. I noticed the problem after being away from home for about 10 days, when the Rocket set idle. Normally I pull two triple shots per day and about 20 total shots pulled per week. I backflush about every two weeks. The problem occurred after about 18 months of use and about 1,300 shots pulled (based on the counter on the grinder).
I found this thread 3 way valve not preventing sneeze on E61
with a diagram of the exhaust valve. I took it apart and found it a little dirty and stained inside, but not gummed up. Even so, I wiped all the parts off, including wiping inside the barrel. I used a pencil wrapped with paper towel to wipe deep inside.
After the disassembly and cleaning, the exhaust whoosh is back in full force. It's a 10 minute job to dissemble, clean and reassemble. I now know how to solve the issue.