There may be minor physical problems at the points where your device connects to your cradle/cable, or where the cradle/cable connects to your desktop computer, that interfere with successful HotSync operations.

Is the connector on your cable/cradle firmly seated in the port on your desktop computer?
Sometimes a loose connection can interrupt a HotSync operation. Ensure that the cable/cradle is firmly attached to your desktop computer. If necessary, detach the cable/cradle, and attach it again.

Is your cable/cradle connected to intermediate hardware?
Some USB hubs, docking stations, port replicators or switch boxes can interfere with a HotSync operation if they are not firmly connected to your computer. Make sure that all connectors on the chain between your handheld and your computer are firmly attached. If the problem persists, try performing a HotSync operation with your device connected directly to your computer. If it works, you can be sure that the intermediate hardware device is at fault.

Is your device firmly connected to the cable/cradle?
When you set your handheld in the cradle, you should feel a click. All rechargeable devices have lights that illuminate when they is properly positioned and charging in the cable/cradle. If you've confirmed that the handheld is properly seated and you're still having difficulties performing a successful HotSync operation, use one hand to apply moderate pressure to the top of the device as it sits in the cradle while you conduct another HotSync operation. If this operation succeeds, contact Handspring for assistance in obtaining a replacement cradle.

Do you tap the onscreen HotSync button when you want to perform a HotSync operation?
Sometimes in doing so you can unseat your handheld. Try using the physical HotSync button on the cable/cradle itself (note that some travel cables do not have this physical button).

Do you think the HotSync button on your cradle may be broken?
Try conducting a HotSync operation by tapping the HotSync button on your handheld's screen while it's connected. You can get to the HotSync button by first opening the Applications launcher, and then tapping the HotSync icon. Then tap the large HotSync icon in the center of the screen.

Are the metal connection points on the bottom of your handheld dirty?
If the connectors are obscured by dirt, oxidation or oil, use a soft eraser to rub them clean. Wipe away any eraser remainders before attempting a HotSync operation.

Is the cable near any strong source of electrical or radio noise?
Move any cell phones, computer power supplies or similar devices away from the cable.

source: pal.com