I really doubt that IMG was more selective with Montana's castings than they were with Yumi's, for example. There really is no reason to be. Montana is not a brand new face nor did she have momentum from campaigns, eds or from doing HC shows in January. I'd guess that they were sent on a lot of the same castings and not all of the shows Yumi walked in NY were A-list.
Sometimes it is just difficult for success to carry over from one market to another and especially from one season to the next.
She already turned down Next before to pursue a contract with IMG, a decision that was applauded at the time... I'm not so sure they'd be eager to take her on at this point, but who knows. There are a lot of other agencies, but then again she had some reason to go with IMG.