I believe it is because the two edges your selected do not touch. The vertical line is interrupted by some feature and your selection doesn't reach the sharp point. Try selecting the short vertical edge that actually touches the chamfer edge.
It is possibly not. But there is an work around to take the Chamfer dimension in it.
1. Rt.Click the view which you wanna to take chamfer dimension.
2. Click "Lock View Focus".
3. Goto "Sketch Tab" > Click Line > Draw Line at the edges where you wanna to get chamfer dimension.
4. Goto "Smart Dimension" > Then take "Chamfer Dimension" of the drawn two lines.
Thanks and Regards
E G S Computers India Pvt. Ltd.