Firstly restart your panel. If this doesn't help you can try clearing the cache. To do this select settings from the CommBox home screen and select the spanner and hammer for system settings.
Arrow down to 'Application Management and open the CommBox signage app. From here you can select 'Clear data'. Restart your panel.
If this doesn't work try uninstalling and reinstalling the CommBox signage app. This can be done from the same settings/application management menu. Select the CommBox signage app and select uninstall. Restart your panel.
Check how long your total playlist is, sometimes it may be too big for your panel to handle, break it up into multiple videos.