There are at least three things to consider: herbicides, insects and availability of water. Let's take the last first - why did the Ficus have to go? If it was because of dry soil then a buttonwood will be a backward step since buttonwood is more associated with mangrove and wetlands than a dry spot. Once the initial flush of growth is over it will require water. A number of insects can be responsible and that you can find by close inspection of the foliage and roots. As for herbicides, this should be easy to consider since presumably you would have intimate knowledge of whether any roundup or broadleaf weed killers have been used, or even fertilizers for grass containing herbicides.