Treo 650 wont sink
Assuming you are running Palm OS and not one of the rare beta Windows Mobile versions, here is my take on this.
Are you using a USB or Serial Sync cable? With either solution, make sure that the setting on the Palm Desktop Hotsync Manager is set to the correct setting (either USB, Serial, IR, etc). Conflicts between the cable type and the settings on your PC are common.
I prefer using USB cables since they are faster and you can tell if you are connected quicker than with a serial cable. Initiate the sync via your phone or the Palm Desktop Manager software. I have had faulty cable sync buttons and this was a good solution for me. If you haven't tried a new sync cable, you might want to.
Actually if you have time and a laptop, you can take your Treo to a Palm store and borrow one of their cables to try and sync. They were really helpful whenever I went in there.