Not really their fault as I am one of them. I brought two cars to the RFID installation at Kelana Jaya there. Both cars, tried at the headlamp and windscreen and their detecting machine couldn't detect. Windscreen they blamed my tint and headlamp they said too close to metal so they actually told me to use TnG card or Smart Tag. So I put the sticker on the bumper and asked them to check with their detector and it worked best (this is for a CRV). They told me that they aren't allowed to officially put the sticker there at the bumper so they asked me to do it at home instead.

For my other car, all places including the windscreen, headlamp and bumper didn't work at all. So after 5 tries (yes RM38x5 spent), the last and final bit worked at the number plate. Each try, I would try first at the installer at Kelana Jaya, then at the actual toll locations (penchala, kerinchi 2x, Sprint). Best for my sedan seems to be the number plate

ayam just installed one on windscreen today, no issue passing thru at all.