... costs around $45 but it's a combo unit. It tachs 2, 3 & 4 blade props as well as being a digital voltmeter for Tx and Rx.

The problem I'm having with it is that I like to use it at home in the evenings when running up engines. This is not the best time to try and use an optical tach. If it's too late or cloudy then the reading become erratic or you get no reading at all.

With cloudy conditions and a grey plastic (APC) prop its no wonder the tach can't see it..

These tachs ( all of 'em, not just H9 or Glo Bee) require good strong light and the light source needs to be the opposite side of the prop from the tach. You also might need to find a way of providing good contrast by painting the prop for example.