If you are using the USB docking cable connected to the same USB port, when you connect the iPod touch versus the "other devices," the problem may dock connector on the iPod touch. What are the "other devices"?
When you say "charging on its dock," is that a different power-only connection? Or is that dock connected to the USB port on your Mac with a docking cable? If it's connected to the Mac through that dock accessory, what happens when you connect the iPod touch directly to the docking cable?
If you have another USB docking cable, try it with the iPod touch using a direct connection to the Mac's USB port (no dock or USB hub).