Samsung commercial displays have a power schedule built into the display settings, separate from anything Screenly controls. A display that is switched off on its own timer shows nothing, however healthy it looks in your dashboard.
Check the on and off schedule
In the display settings, find the on and off timer and confirm it matches the hours the screen should be showing content.
This is worth checking on every display in a fleet rather than one, because the schedule is set per display and factory defaults vary between models and between the settings a previous installer left behind.
Note
A screen that is powered down on schedule reports as offline in your dashboard for those hours. That is expected, and it is worth knowing before you set up alerts for screens dropping off.