If you use the / m flag with msbuild, I saw that this causes problems if your solution and the project dependency structure are not entirely correct. I found this post that looked interesting, and maybe this will help you.
I looked at my solution structure, and I had a WCF service project that was not properly added to the solution. I removed the service and added the service again, and I was able to build with / m: 2 using 2 procs. I found that the error you see during the build process will point to the source of the problem.
source
share