Try upgrading a couple to a higher ilvl, I think that's what triggered it for me. Or maybe every 5-6 100's.
Though I'm pretty sure you need at least 3 100's to spawn higher missions. Also, having someone in the 90-94 range will help spawn those lowbie missions. Most of the time I see it happen is on brand new characters. Clear out the existing missions and keep a 90-94 around and it should spawn more.
Get some more followers (there's a lot to be gained in the wild from very short questlines), that should automatically increased the number of missions you get. I have 6 chars with maxed out garrisons, and on all of them I can check in every ~3 hours or so, to complete missions and reassign without ever having more than a handful left idle.