It could be a number of things. I installed 2 nucs two months ago. One took right off, building and growing. I added their 3rd box 3 weeks ago and they're ready for a 4th. A new medium every 3 weeks isn't bad.
The other hive, while busy, wasn't really growing. I added a box 6 weeks ago when I added one to the other hive. I actually pulled drawn frames from the other hive and put them into the weaker hive to help them catch up. I checked on them today and they exploded in population. The box is completely built and they're ready for their 3rd one.
You could have gotten a slow starting queen. Check the brood, if it's not a solid pattern, you may consider requeening.