There are many and varied reasons for Active Sync problems.
Have you looked through some of the solutions in the
Active Sync Forum?
Have you,
Soft reset your Ax
Rebooted your computer
Installed any new software lately (some software conflicts with AS)
Checked that your firewall isn't blocking it. (try turning it off)
installed any updates to windows ( can change settings, including the Firewall)
installed any new anti virus software (same as above)
Uninstalled Active Sync and reinstalled it (rebooting the PC in between)
Deleted the partnership and re-established it (Rebooting the PC again.)
Tried a different USB port (main not Hub)
Cleaned the connectors on the Ax and the Cradle (a pencil eraser works quite well)
Held your mouth the right way (critical)
These are a few of the things that can cause problems.
It's somewhere to start.