session files not being deleted

I have session files that are not being deleted within two of my multisite installs (separate cpanels), and need some help figuring out what might be causing it. I try to keep the settings for my various multisite setups the same, so am a bit confused about what would be causing the issue on just two of the installs. But maybe I changed something and forgot about it. Any ideas as to what might be causing this, and/or what settings to check, either within wordpress or cpanel or someplace else?