Cappy...double check the device in Measurement and Automation Explorer. It certainly sounds as though your 6024E is configured for differential mode. To correct this, launch MAX, expand the Devices and Interfaces Directory, locate your device, highlight and right click. Choose properties. There will be several tab views. Choose the tab "AI". Within this tab, you should see that you are still configured for differential mode (default). Select referenced or non-referenced single ended mode and you should have access to the remaining channels.
jerry