If teeth were missing, it would probably make a clunking/grinding type of noise. This 'slipping' you describe makes me think that the starter drive gear or 'starter clutch' is not working properly, sticking or seized. Have a look at these drawings and notice that the starter clutch is not included with the starter, it is separate.
http://www.bikebandit.com/houseofmotorc ... 0k/o/m7931
Now have a look at this view of the starter clutch. See that shaft (#4), spring (#10), and gears (#5 & 10).... they all need to slide in and out as the starter spins to engage the flywheel. Check to see that yours do slide easily.
http://www.bikebandit.com/houseofmotorc ... 1sch228497
If you need some of these parts, that site the drawings are on should be able to set you up.