I realized this, think of follower ilevel as a another "tier" possible to get quests from. From what I observed, if you have only followers in one level tier, you get substantially less quests. If you have all followers level 90-94, like at the beginning of the expansion, you only get like 2 quests at a time. You did not notice it, or care much about it at the time but you can notice this on an alt if you keep followers in that leveling range.
So there are several leveling brackets
As you can see, if all your followers (or most of them) are level 100, but are not upgraded, you only have access to 1/4 of the level 100 quests. So it seems like you are getting nothing.
I suggest to get a level 90-94 & a 95-99 follower in the meantime, get salvage yard, put work orders in war mill/dwarven bunker and spam low level missons to get follower upgrades. Get 3 level 100 followers to ilevel 645 and you should see the normal 10-15 missons a day you saw while leveling.
I had this problem, got a few followers upgraded and now im swimming in level 100 missons. As a plus, some of the really higher leveled ilevel missons give like 20k exp, so if you bring a level 90 with it, they still get 10% exp and you can level them quickly that way. It sucks for people who do not understand this system, because if they do not plan accordingly, they will hit a brick wall of 2 missons a day.