A couple things.
In beta, the Rich yield missions would always give purple craft materials. Unfortunately, they nerfed them shortly before release so that, yes, they now are quite a bit harder to get even with Rich yields. It's not just Diplomacy.
Also, missions do have a cooldown of sorts and multiple companions can never be on the same mission at the same time. Once you use it in the list, it's "consumed" for a bit. Even if you cancel it, it won't necessarily cycle back into the list.
And, third, as far as I know the Diplomacy missions aren't weighted in either direction and your mission lists just get populated somewhat randomly depending on your skill level. But I wouldn't worry too much about it because the morality shifts from them are pretty miniscule unless you're pumping out a lot of the high grade missions of one morality.