Restaurants that are not chains are small businesses run by an owner or two with a lot of hired help. No small business will operate smoothly 365 daysa year. Your lucky to get a full week in without some problem showing up. There are many things that can and will go wrong in any business. In the restaurant business there is even more that can go wrong from food supplies to help not showing up. Imagine Saturday night at a busy restaurant. The cook quits or calls in sick, a couple of wait staff don't show up. Maybe your supplier was out of your Saturday night special or sent the wrong thing. Any of a thousand things can go wrong but you keep everything running smoothly most of the time. Mistakes occur even under the best of circumstances because there are humans involved in the preparation of food. If a customer complains it should be taken care of. If it is not, it is a down check. It is possible that the owner or manager was not made aware of the problem.
I always give a caveat when recommending a place. I always say it was great when we were there or what we had was delicious. No two people have the same taste buds. I know this because my DW loves Salmon.