I was on the Liberty in March and overall we had good service in the MDR. We were also in the Silver Dining Room. We were a party of 12 and I anticipated issues because we were Anytime Dining (or whatever it's called on Carnival) however we only had slow service one night. They even were able to seat all of us together by pushing two tops and four tops together and the longest wait was about twenty minutes and most nights it was immediate or less than five minutes. That I didn't expect and I was happily surprised.
The one night we waited for a very long time between the main entree and dessert. We would have just left but it was my brother's anniversary and we had ordered a cake. The delay was because the waiters had to do their song and dance act and that seemed to throw their rhythm off.
I know how you feel when you have had great cruises and then have friends and/or family join you only to experience a bad cruise. You somewhat feel responsible though it is not your fault.
I do hope that you do try another cruise and perhaps try a different line or ship. In my thirtysomething cruises the good experiences far, far outnumber the bad.