What model iPod and iOS works and which does not?
Sometimes a 4G iPod will not fully sead in the dock because its case configuration is different that previous models.
Also, sometimes an iOS update breaks compatibility.
Have you checked the dock manufactuer's support stie for compatibility?
Sometimes this works:
- Reset the iOS device. Nothing will be lost
Reset iOS device: Hold down the On/Off button and the Home button at the same time for at
least ten seconds, until the Apple logo appears.
- Reset all settings
Go to Settings > General > Reset and tap Reset All Settings.
All your preferences and settings are reset. Information (such as contacts and calendars) and media (such as songs and videos) aren’t affected.
- Restore from backup. See:
iOS: How to back up
- Restore to factory settings/new iOS device.