Unfortunately you didn't provide much details about what you are doing, so I can only give you a general advice.
If you are scanning the current subnet, make sure the
ARP query lookup method is enabled under MAC resolution in the Options, on the Additional tab:
If you are scanning a different subnet, try ticking
Allow ARP outside current subnet on the General tab:
Also, you may like to have a look at this article, which provides an
overview of additional MAC addresses resolution methods for non-local subnets.