Instant3D is a feature in SOLIDWORKS which makes it easy to use the mouse to manipulate features in a model by pushing and pulling them.
One of the behaviors that is changed by Instant3D is what happens when a dimension is selected, if Instant3D is enabled, and the text on the dimension is selected, then SOLIDWORKS starts editing the dimension value and the dimension is never selected.
This means that, when Instant3D is enabled, you can only select a dimension by clicking on the dimension lines which are harder to select.