I wil try out a few combinations of generals and troops...
This was in version 1.09, with the leader and RR mods, i'll look up the name of the General and try to duplicate the "occurance".
Edit: the General was Charles Hamilton. I had the leader attributes set to random, he happened to be an amazing 5-3-3 leader that game...
Ok, new game, got it happening again: in the early june turn I combined Hamilton (actve) with the Washington Brigade, the USMC and the Harpers Ferry guard. Could combine and disband as much as I wanted, in any and all combinations.
In the late june turn Hamilton (inactive) can only combine with the Washington Bde. Adding any other unit (USMC, Harpers Ferry Guard and the newly arrived 1st MD Brigade) makes the + grey out. Even selecting only Hamilton and one other unit does not give the +. The only option for combining is Hamilton with the Washington Bde. Between the turns the division was disbanded.
I can not try out the early july turn, as Hamilton got wiped out at Harpers Ferry. I just got him some troops to play with, didnt look at what Athena was doing. Oops.....
Ok, kept trying to find odd things. I first tried at Alexandria. All activated leaders could combine after being prooted to division command. A non-activated general could not be promoted to division command, but could combine with one unit to form an independend brigade..
But I disbanded a few divisions, and now I can not form them again. Neither the inactive general, demoted, can form an independend brigade, nor the active General, with division command enabled, can combine with anything. The combine button lights up in this case, but clicking it does nothing...
Got to say that I fooled around a bit with William Nelson in between. First forming an independen brigade, then disbanding again, and then trying to see if he could form up with the militia (this is in Cincinatti). He could not, but after this the + button stopped working, and he could also not form up with the brigade anymore...
N. Lyon on the other hand can form an division, so it isnt absolute... (and disband, and form, and disband...)
It isnt always the case, but it seems inactive leaders usually cant combine to form divisions. But sometimes they can (especially directly after disbanding one). And active leaders are usually ok with combining, but sometimes after fooling around with inactive leaeders they can not combine anymore.