As Rich said, this is because the plastic teeth have broken or worn down to the point where the electric motor is not gripping it any further. You do not have to buy an entirely new antenna to fix this however, just a new mast. These are fairly easy to find for that reason, and also because many owners of all makes/models with retractable antennas neglect to lower the antenna when going through a car wash, and the overhead brushes rip the antenna right out. Google around for "power antenna mast" and find one that fits the measurements. Namely, the height of the mast and also the height of the tallest "step" (which determines how deep into the power unit they are when it's down), perhaps also teeth spacing and size, although these may be a standard size. There's plenty of tutorials online as well, just Google around for "rebuild power antenna".
Not only is it much cheaper than replacing the entire unit, but it's also very easy to do, and doesn't take much time at all. Worst case scenario, you get another power antenna like you were planning anyways, but I would at least attempt a rebuild if I were you, since this sounds like exactly what's going on.