A couple of things to watch
1) Make sure the flask is centered on the stir plate, if not the stir bar will be thrown out to the side (centrifigal force) instead of spinning about itself.
2) When you put the flask on the stir plate tilt it slightly to get the bar to slide to one side; allow the stir plate magnet to "grab" the stir bar, then position the flask in the center.
3) Start slowly
4) I use a 1" bar on all of my starters. I have used a number of containers, a 300 ml jars, qt. mason jars and 1000 & 2000 ml Erlenmeyer flasks.
Good Luck. David