It’s also possible your nuc has a bad varroa infestation. That really slows the bees down.
Nuc may have what ever, if a beekeepr does not notice.
Forexacple the British use much mesh floor in nucs. That will not work.
3 frame nuc nees only 2x2 cm entrance and nothing more.
I have splitted polynucs to 3 frame nucs. They build up amazingly well.
If you make a nuc in home yard, foragers return to the home hive. It takes 2 weeks that nurser bees becomes foragers.
When nucs has special backround they are often short of pollen.