Never had that issue, but is there any chance your are hitting the paddle shifter by mistake and are putting your care in manual mode by mistake? You could be waiting for it to shift when it is waiting for you to shift. Dunno, just guessing.
Mine 2012 smart pure(451)will usually stick in first gear or second when I clime a hill or so. The trick I found is to switch the car into manual when it trys to stick and shift that way then when your revs are low enough or you have climed that tough hill then switch back into auto mode.
Why do I do that...to avoid reving above 3K-4.5K RPMs and it helps the transmission out when it comes to the difficult things.