Yes, it appears to be the wire. I think you mentioned it is Romex, buried in outdoor masonry. Romex is meant for inside building walls and is not meant to be exposed to moisture. Whoever selected it chose the wrong type of wire for the application and that's probably why it's failing now. I think you need what is called type UF cable meant for outdoor direct burial or you'd need to install a true conduit run that connects to each box. Good luck.