Each mission type has a respawn time which is higher than the mission time (when used repeatedly). On UWT you will notice that if you spend so much time sending out companions on fabric missions you will then start getting more and more gift/metal missions. If you send out companions on only companion gift missions eventually those will go away and will pop up with more metals/fabrics.
This is easily proven false as very frequently as soon as a mission is finished you can send a companion back out on the same mission.
That being said, a significant part of Diplomacies problem is it is *still* broken from when they went through and divided the missions up republic side vs. imperial side based on the flavor text. IIRC for grade 4&5 there ARE no rich (and possibly bountiful?) medical supply missions for the empire. And vice versa for the republic for grade 3. However, I believe both sides get all the gift missions, so for diplomacy, at least, IIRC there really ARE a lot more gift missions available in the random pool.