Jason,
If none of the above mentioned techniques work, what you can do is to wait a bit longer so that the air in brake pipe gets through the entire train. What you are saying is most likely correct, the coaches have their brakes on.
Do the following:
Once you are coupled with the rest of the train wait few min, i.e. 2 or so with the brake handle in driving postion!! I know that when you change the cabins, the handle goes automatically into the cutoff postion. In this postion you can wait all you want and the brakes will not be realsed.
The 2 second wait should give you enough time to have the brakes realesed on coaches. ALso it helps to put the brake handle into the "quick brake realese" for a second or two (few times).
Good luck