You gotta listen to your tanks!
I learned from Fred Eckhardt that I have to listen to my beer... but I learned from Brock (my boss) that I have to listen to my tanks! We had a rather recalcitrant batch of Elissa that just didn't want to carbonate. First we hit it with the CO2... apparently not hard enough. Then we discovered a sticky (and consequently underperforming) pressure relief valve, which was cleaned and returned to service, but then Brock started listening to the tank an decided the CO2 wasn't flowing.
Our older CO2 "stones" apparently weren't as "fine" as our current ones, but being "fine" means it takes a lot of pressure to push the CO2... say 120 psi! We knew it was flowing when the stone started "whining" in the tank. In short order, our CO2 level was climbing!
After six months, there are still new tricks for this brewer to learn!