It could just be a reception problem, usually the dispatches are using a tall tower to broadcast, the crews are on the ground, possibly transmitting with less power as well.
What kind of antenna are you using?
I have also seen in some areas where the dispatcher and crew are on two different AAR channels. Does your scanner have the ability to scan the entire railband? I programmed all 97 AAR channels into one of my handheld scanners so I can scan in areas when I'm not sure what channels are being used.