Hall Effect Sensor for POV Project
From the datasheet of the hall effect sensor, the thing is only checking for 125usec in every 125msec - so if your magnet whizzes by in &lt;100msec it will miss it.

Obviously you haven't stated the speed your magnet passes your sensor, but as it's only active 0.1% of the time, this seems the likely reason to miss.

Can you put it nearer the shaft/axle ?