it could be helpful to know, which LANDesk version you use...
You can change some settings of the Local Scheduler on command line, s. About Local Scheduler Command Line Parameters
If you restrict the time range on the scans, it will only run during those times, so you want to either not use a time filter, or set it for during your times of operation when you know your machines are online. With 18,000 clients, I would recommend:
Set scans to run daily.
Set no filter, or a filter from 6am to 8pm, common daily operational time range.
Set a random delay of say, 2 hours or so - you don't want all 18k clients hitting the core at once.
This will set your clients to run daily, during the time of most availability, and will make it so each client delays from 1 to 120 minutes before kicking off the scan. The delay is there to help ensure that all 18k machines don't run at once by staggering them out.