A) Make sure that Channel Schedules are configured right in terms of start date and times.
Go to Channels.
Select the Channel that you are using.
Click on the Schedules tab.
See the list of schedules, and control their start date, time, and time zone.
B) Make sure there is no mistake in terms of AM/PM settings. OMMA uses 24-hour time format. For instance, to select 1:30 PM, you should click 13:30 on "Starts" field.
C) If you manage timing by using Campaign Dates or Time Range functions in display rules inside the playlist, make sure start/end dates and times are correct. Also check AM/PM settings.
E) Make sure your device is connected to the internet.