scanning 0 files

I am having an issue with scanning and I was wondering if anyone had any suggestions and/or experienced the same thing.

I have computers segmented by policies and each of those policies will have a different weekly on-demand scan depending on the needs and risk for that group of computers. I noticed the issue when reviewing reports - I can see where some of the computers in the policy scanned a high number of files and another computer in the same policy scanned 0.

This is happening across the scope of most of my policies and computers. What reasons would cause a scan to initiate but not start?