[Newbie] Magnetic DRO Problems

[Baja_Dude]

My lathe DRO has a spot along the travel where is stops reading values. No matter which end i start at, the DRO begins reading normally, then, in the same spot, the values stop changing and then resume when past the 'dark zone', which is about a 4 or 5 inch section.

Is this an indication of contaminants on the glass?

I just replaced the reader head because it was not reading values anywhere along the travel.