The subprocess handling is rumored to be problematic especially on OS X. Unfortunately, it seems to not be known what exactly the problem is. There are several open bugs relating to it. One suggests changing a timeout helps, and another that setting process-connection-type
to nil
helps. Please read them for details about where and how these are supposed to be applied in order to help.
You can also try the "Mac Port" which IIRC is supposed to have better process handling, and in general be very nice.