There's a little helical gear (sorry, I've taken mine apart but I can't remember the terminology for some reason), and if it's dirty it can have trouble engaging. I cleaned mine with WD-40 and used some silicone spray to keep things nice and slippery. There might be something better for it, but it's been going strong for about a year or two now. The problem I had was that the previous owner put in a bolt of the wrong thread pitch into the ALUMINUM transmission housing and stripped it out. One day, it got loose enough that it just spat the bolt onto the driveway. I had to retap it with a proper bolt.
