Sounds like you're not getting data across on the send pin from the N900 to the PC. It could be a dirty contact, or the dreaded USB disconnect problem starting. How old is your phone?

One thing to try would be to check the cable you use and make sure it's clean on all it's contacts. You may also want to VERY GENTLY examine your phone's USB port and see if all of it's pins are clean. (You'll want to focus on the middle 2 pins (middle 3 on the micro). If it's even slightly wiggly, and you're still in warranty, take it to a repair center to get it fixed.