Are you using LDMS 8 or 9? Also, what is the exit code you get? In 9.0 that can be found by looking at the task results in the console, on both 8 or 9 that can be found by looking at the sdclient.log file in the LDClient\Data folder on the client machine.
If the command works in a command prompt, then it should work from LANDesk. If you are using 8.8, you may need to use a batch file instead. If from 9, then you have to be a little "creative" when modifying the command line. Just move the commands around some until it works.