Everything goes well until I come to the 'Gathering of the Armies'. I am able to merge at Henry all except the detached brigades of the Nashville force (eg. 1st Col. Brigade, Kentucky Volunteers, Nash Artillery) into Prentiss's 6th Division of Sherman's Corps by the 'drag & drop' method. All are unlocked. I cannot get them to merge.
I put them into Sherman's corps & try to drag them into the 6th division but it does not work.

I would appreciate any & all advice.
Wonderful game!