Samsung
Change USB Connection Mode auf Samsung
The first time you plug a Samsung phone into a computer with a USB cable, a pop-up asks what the connection should do: transfer files, transfer photos, or just charge the battery. Whatever you tap gets saved as the default for every future connection, so the pop-up does not appear again on its own.
This is why a device can suddenly seem to stop syncing files: it is not broken, it is simply set to Charging only from a previous session. The fix does not require unplugging and replugging repeatedly - the mode can be changed at any time from the notification shade while the cable is still connected.
Where to change it
Follow this path: Swipe downwards to open the Notification shade ➔ Tap on Android System ➔ Tap on USB for... ➔ Choose an option.
The notification only shows up while the USB cable is physically connected. If it is missing, check that the cable supports data transfer and not just charging, since some cables are power-only.
Switching modes here does not delete or reset any saved preference permanently - it only applies to the current connection, and the phone will remember the newly selected option for the next time the same cable is plugged in.