Hmm.. been a while since I looked at this. Too long it seems, as I can't remember what the problem was.
If I am right, the units in question was wayting for other dismounted units in the hierarchy to catch up.
Managed to complete the scenario during the weekend. But had some further issues with mounting.
One improvement I would like to see, is some sort of error message telling why a mount/dismount order cannot be carried out.
I have a feeling that sometimes the dismounted units are stacked - the icons are overlaping so that a single unit cannot be selected for mounting?