I would not delete them all, as ConfGen said try highlighting “refresh device information”. This should cause the device to check back in to the WDM server BUT BEWARE, THE DEVICES WILL REBOOT WHAEN THIS HAPPENS as the IP will change client side. This means a reboot to disable the write filter, so do this out of hours.
If the devices still do not show up I would say there is an issue with the server and you need to fix this. If you delete all the devices it could be painful trying to discover them all again,
Cheers,
-TT