Use hardware threads as independent cpus changes Barrier() behaviour

I am using Open MPI and C++ to try out simple programs on my laptop. When I use the 2 cores on my computer as processors, the program runs us expected. However, when I instead do –use-hwthread-cpus, it seams that I never get past the Barrier(). That is the only thing I am changing, just the way I start the program. It is the same .exe file running.