Some basic forensics on a device is probably your best bet for getting to the bottom of this. You're going to want to look at the vulscan.log file on a device that is having this problem. It will reveal in more detail what might have gone wrong. You may also want to review other logs on the same device.
on XP, look under C:\Documents and Settings\All Users\Application data\Vulscan (keep in mind that "Application data" is a hidden folder)
in this folder you should see several vulscan log files. The most recent should be vulscan.log. vulscan.1.log will be the next most recent, and each greater iteration will go further back in time.
and under C:\Program files\LANDesk\LDclient
there are many log files in this directory as well that might provide some insight. Sort on type, and look for all the .log files. Review them where they seem relevant.
Also in this folder you should have the SDMcache folder which should contain any files downloaded from the server. Look in this folder and see what is or is not there. You might find some files that start with *partial*. If you see these, and there is nothing running at the moment, that's a sign that a download didn't finish. These files should only exist while a download is in process, but if the network was interrupted during the download, it might leave a partial file. LANDesk is smart enough to restart a download, so the partial file might not be a terrible thing, but its a clue at least.
These are all just tips to help you figure out what is going on. Since you're not seeing the problem on all devices, its probably device specific problems either relating somehow to the network, or somehow to a localized problem like disk space on the device, file corruption, or something else like this.