Hi dingding
Are you running the most recent Firmware Version on your Airport station? If not, I suggest you perform the update.
Then just go into all the settings and maybe deselect them one at a time, then quit both iTunes and the Airport app. Then relaunch them and put back the settings. This might jog things around a bit and have a positive effect for you .... it's worth a try. I think if the Firmware update doesn't work, then removing the speakers from the equation, then adding them back again might just do the trick
I have no experience with remote speakers as I am connected to a fully wired system.
Here's how to update the firmware:
MacintoshHD ~> Applications ~> Utilities ~> AirPort Utility.app
Or the following Apple website:
Apple Support Downloads
.