Actually i kinda found a trick to make the scanner easier to detect my RFID. So far it works for me 10/10 once i've started using this trick.

It is to go as near as possible to the right side when driving in to toll.

It seems to me that if you drive too much to the centre or to the left, the scanner somehow cannot detect our RFID, maybe cos of too little gap from the scanner or something. So far it works for me 10/10, without fail. The RFID sticker is on my car's left headlight.