Vortex Range Finder issue - Impact 1000

[creekdonkey]

I have an Impact 1000 that will not range dark colored targets at any yardage. I was at a 3D shoot a couple of weeks ago and it wouldn't read some of the darker targets. Borrowed a buddy's Leupold and it functioned fine.

Is this a known issue with their rangefinders or is mine potentially damaged or defective?