What is happening here could be a few things:
Vulscan could be running at the time that you are trying to do a Software Distribution job. Vulscan and Software Distribution sdclient.exe share the same mutex so they don't run at the same time.
Also there could have been a job that didn't clean up properly...
So definitely look for running SDCLIENT processes and running VULSCAN processes.
If the Vulscan client is still running there may be an open dialog that hasn't been answered, or a scan is taking a long time.
A vulscan that is still running will show a vulscan_PID_###.log file in the C:\ProgramData\LANDESK\log file in version 9.6, in version 9.5 this will be located in C:\Programdata\vulscan
You can look in those logs to see what it is currently doing.
One of the causes of this could be the fact that WMI calls were introduced into 337 of our definitions and it significantly increased scan times for Vulscan and caused 100% CPU usage at times in TrustedInstaller.exe (A windows process).
Our Patch Content team has removed the WMI calls from these 337 definitions and pending testing they will be released into content. This should significantly speed up the scan time and CPU availability.
Check your schedules and see that Vulscan is not scheduled to run at the same time you plan your scheduled jobs. I think you will see that a lot of users try to wait it out, but it is not guaranteed that those processes will clear out on their own.