You can set breakpoints to specify the places where your program should stop. Parallel program stops only if all processes are stopped. Imagine the situation that not all processes have reached some breakpoint – the breakpoint may be set in the body of some condition statement. What happens with the rest of processes? Some of them may run until they finish execution, other may reach other breakpoint or watchpoint, or wait at communication or barrier point. A parallel program stops only if all of its processes stop for one or another reason. To make sure that all processes will stop at breakpoint you should specify the place, which can be reached by all processes.
For more detailed information on breakpoints consult the topic Breakpoints overview.