FASTag for payment of toll on National Highways has been made mandatory from January 15, 2020. Nevertheless, its usage has already been started and if you too confront a situation where in the scanner at a particular toll plaza does not seems to be working i.e. the toll amount is not deducted from the RFID enabled FASTag on your vehicle, you can pass through that toll for free.
NHAI under G.S.R 427(E) in the ruling mentioned in The National Highway Fee Determination of Rates and Collection Amendment Rule 2018 states "Provided also that if a vehicle user with a valid, functional FASTag or any such device with sufficient balance in the linked account crossing a fee plaza installed with Electronic Toll Collection infrastructure, is not able to pay user fee through FASTag or any such device owing to malfunctioning of Electronic Toll Collection infrastructure, the vehicle user shall be permitted to pass the fee plaza without payment of any user fee. An appropriate zero transaction receipt shall be issued mandatorily for all such transactions".
So, beginning January 15, 2020, ensure you have a RFID FASTag affixed on your vehicle's windscreen as without it you will be charged double the toll amount.