There could be a few reasons...
firstly - I presume your measuring for AC voltage
secondly... this is not a very optimized alternator, at least not the way we built ours. To get even a couple volts from ours, we had to use two coils in series, each coil has 400 windings of #30 gage wire. So... for the speed your running at, you should probalby be using lots thinner wire, and lots more windings. The hole in the coil should be about the size of a magnet, and the outer diameter should be about the same as the distance from teh center of one magnet to the center of its neighbor. The coil should be wound as thin a possible, as the flux drops off rapidly as you get away from the magnets.