Some servers don't have their volumes being monitored and some do.
Is there a reason this would happen?Not a new environment and not new servers.
Is there a global setting to default add all the volumes for all servers?
Could be the techs havent added them when adding them to SW or the System account has removed them as a UID has changed at some point. There is some discussion of that in this thread: (+) What nodes don't have their disk volumes monitored? - Forum - Network Performance Monitor (NPM) - THWACK
Run a network scan of your server subnets to detect the missing volumes is probably your best solution.
I occassionally see this occur when a server is rebuilt or has gone through a P2V migration and the deployment engineer failed to scan the new device resources assuming that the old resources will update automatically. What method are you using to detect the devices that are not monitoring volumes?