For my ex car with VKool, using Smart Tag was a touch-and-go situation (no pun intended). I always had to drive very slowly, put the Smart Tag close to windscreen (I noticed the higher the Smart Tag on the windscreen, close to the rearview mirror, the easier it got detected). And I had to start placing the Smart Tag a few car lengths from the gantry or it would not detect.
Despite this, there were times the barrier would not lift and I had to wait for assistance, or reverse back and move forward again if there was no car behind me.
The easiest way to overcome this was to wind down my window and dangle the smart tag above the car roof - yeah, not glamourous.

FYI, I have friends in Singapore who have to cut a "hole" at the location where they mount their In-vehicle Unit (something like smart tag used to pay for road charge and parking).