When you installed your NUC, the bees had gone through a lot – Transportation, searching for the queen, a detailed inspection, being introduced to a new box that does not smell like home, and much more… Anxiety is high and perhaps the weather was not too warm.
The entrance reducer should be removed one week after moving the bees in as long as the night temperature is not dipping below 50 degrees. During a nectar flow, the reducer will only cause a jam at the entrance of the hive. Every second counts for your busy bees, so remove it and let them come and go as they wish.
In August, when the dearth is on, bees don’t have much to forage on. Larger bee colonies will attack smaller ones attempting to rob their honey. Also, during that time, you will see wasps and yellow jackets trying to steal the honey or attack your bees. At that point, you should reinstall the Entrance Reducer with the smallest opening to give your bees a chance to defend the colony. It is much easier to defend a 1″ opening than it is to defend a 20″ opening.