Yep! I have an idea!
You are going to have to modify the line in /etc/fstab (on boxB) to look something like this:
<server>:/nfs/share /local nfs defaults,user,rw 0 0
On boxA, make sure you export the directory with the same options as you define in /etc/fstab:
Just as a side note, when you're asking about proper configurations in the future, it is best to usually include the configuration file or a snippet from the configuration file in question. In this case, we would have liked to have seen the /etc/fstab from boxB, and the /etc/exports file from boxA.