HTC3600i using Active Sync4.5 syncs 1 time then doesnt the next ti

i have installed AS4.5 and it syncs with the pda phone one time. then the
next time i plug in the usb, it says no connection and AS does not
automatically start even though it is checked. I try to connect manually but
it still says device is not detected.
This is the final part of installing and reinstalling AS4.5 and trying to
sync. it will work the 1st time i plug in the phone to the usb cable, then
the next time i plug it in, AS4.5 doesnt recognise the device.
What could be the problem?