Below is a list of troubleshooting steps for investigating the issue of a phone’s battery percentage decreasing even when plugged in:

1. Check Battery Settings
   Make sure your battery settings are optimized for your device and usage habits.

2. Check if the Charging Cable is Compatible
   Verify that the charging cable and charger you are using are compatible with your device.

3. Run a Battery Diagnostic Test
   If the previous two recommendations did not yield any results, it is recommended to run a battery diagnostic test on your device.

4. Update Software
   Make sure your device’s software is up to date. This may help if certain battery-draining processes are resumed.

5. Try a Different Source of Power
   It is possible that the power source you are using could be causing issues. Try using a different charging cable and charger.