What version of Express are you using? Have you made sure that you are on the most current version? What should I do if I'm having issues with the Garmin Express client?
I'm having the same problem. I had it last year but it went away after an update. The problem re-appeared in the last few days. I'm using the latest Garmin Express software.
If you have followed the steps in the article I have provided I would suggesting trying a master reset on the 310XT, or calling into our support line so we can explore further troubleshooting steps: http://www8.garmin.com/contact-us/
Alternatively, you can attempt a reset on the device: How do I master reset my Forerunner 310XT?
