We would like to understand whether you're using the ASUS original charger. We suggest that you try with new cables and check again.
Please also note that "fast charging/charging rapidly" appears after a few minutes when you plug in your charger. Please make sure to check your lock screen after a few minutes.
Furthermore, there are some scenarios that may affect the fast charging status:
A. If the system is under high resource usage (high temp) or the battery level is over 85%, the device will trigger a protection mechanism, which lowers the charging current.
B. Check if there is some object or stain in the charging port. We recommend to use a soft brush to clean the charging port.
C. We recommend to test charging in safe mode. How to enter safe mode: https://www.asus.com/IN/support/FAQ/1005163
If in safe mode, the problem is not observed, the system performance may be affected by 3rd apps. We recommend to factory reset the system.
If the charging status is still unstable in safe mode, we recommend to go to repair center for further checking.
If the problem still appears, please tell us at which battery levels does fast charging not work.