Do you try to open up / close down the roof whilst driving?
It will only work at very low speeds respectively stand-still.
It works for me never experienced a bug with Convertibles so far.
Yeah I'm doing everything right. More recently I've stolen a car which already had the top down and that worked fine, but when I steal a car with no one inside and the roof down, the convertible button fails to work. Cars that I've experienced the problem with include: Rapid GT, Sentinel, and 9F/R8