Its a long known problem of car tint and Smar Tag. V-Kool especially has this problem and each time you wanna tint your car, they'll warn you before hand. Though my car is VKool tinted, i'm still using the Smart Tag and what i'll do is that i place the Smart Tag unit really close to my windscreen when i'm approaching the gantry and it'll detect it. However, this does not work in all the toll booths and in particular the Sungai Buloh exit and Subang exit has this problem. Last time i used to get stuck in this 2 toll booths till i decided to wind down my windows to get it scanned when i exit. Also what i notice is that i normally get stuck only when exiting/paying the toll booth. It normally won't happen when you're entering the highway.
But if i'm not mistaken, Ecotint's highest range of tint does not have this problem, though VKool have this problem. Probably got to do with Ecotint's low UV rejection ratio while VKool rejects UV pretty well.