I usually set up a 3 wire cable with hardware handshake jumpered out, if you have this or this doesn't cure it then I would imagine that Mastercam is waiting for a s/w handshake command from the Mazak. The programs I use just send with s/w H.S. and that is it.
Is the Mastercam s/w set up to look for some kind of code that indicates recieve is done?
This would probabally be on the Communications set up screen.