have you checked out the help entry found at "Tugger trains, model"? It explains a simple way to create and hitch a tugger-train, and I believe it's a quite detailed description.
It's important that hitchFront/Rear only work in a collision control, so probably your trailers are "directly in front of each other" but not collided yet.
is it possible that you post your model? Or at least the contentslist of the track on which the vehicles drive and the method for creating the tugger train?