I had the same problem with my drivers side seatbelt. I took it apart, ane tried to see what the problem was. My conclusion was that there is a device inside the seatbelt mechanism that determines if the seat belt is in an upright position (level). If the seat belt is at all under stress (i.e. in the case of sudden crash)and the seatbelt mechanism isn't level, this device prevents the seatbelt from disengaging an giving more slack.

If you re-adjust the bolt that keep the seatbelt attached (the one at the bottom as you attach it the door frame) so that it is level, and you also make sure that the hook at the top of the seatbelt is in the right position, it should allow the device to stay level.

This is my theory anyway. Of coarse it didn't work for me because the same time I tried to fix this problem I also performed the "minute mod" and this totally kyboshed my seatbelt spring. I had to order a new one ($180 Can. <img src="/forums/images/graemlins/frown.gif" alt="" />)