Certified GKH Gardening Expert
From your description it sounds like maybe they are receiving too much water. Oddly enough this can happen from a lack of water also. It is uncommon to underwater, though, as these are pretty drought tolerant. Are there any other symptoms?
Here is a link to a few articles that you will find helpful: https://www.gardeningknowhow.com/houseplants/norfolk-pine/