There's a few reasons
Are you using nova launcher? There's a setting to hide status bar
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1. Open Settings
2. Tap Notifications
3. Tap Do Not Disturb
4. Tap Hide Notifications
5. Scroll down and is there a setting enabled called Hide status bar icons?
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1. Are you- the Good Lock app from the Samsung Galaxy store
2. Open the app and navigate to QuickTime
3. Turn on QuickStar
4. Tap Visibility of indicator icons
5. Scroll down to the System icon section
6. Toggle on the icons you want to unhide
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1. Go to Device Restrictions
2. UnRestrict the Status Bar option
3. Click Save to save the profile