The reason is that when the operator is busy, they send any new job to the dispatcher (the output of the operator). The dispatcher doesn't have anything connected on its output, so the job just sits there. If you look at the component list table you will see that the combiner thinks all quantities have been satisfied.
The way you've connected this up makes sense if you want to first give the specified operator the chance of doing the job, but then if they are busy have the dispatcher choose someone for the job - but for that to happen the dispatcher needs output connections to all the operators.