If this really is the case then you have no wind.

I suspect you are reaching cut in much too early and going into stall. I am not sure what sort of ammeter you have, but try something that will measure low current and check that it is really not reaching cut in. If you get a few mA at very low prop speed then it is cut in.

Alternatively read the ac volts on a pair of the bridge ac connections and it should rise with speed. When it starts to go constant at about 9v or so you have reached cut in. If in doubt remove the battery connection and if you are cut in and stalled the speed will shoot up.

If for some reason you are genuinely not reaching cut in then suspect useless turbulent wind or some alternator fault dragging the thing down, possibly a shorted diode.

If it doesn't get a real rant on with no battery connected then disconnect the rectifier and see if it gets away.

If you can establish the cause of trouble then it is easier to know how to fix it.