Sketch dimensions in V15, I place in Part, do not show up unless I move the cursor over them. The lines and arrow heads show in black, with a gap where the number should be.
I.e. |<--- Blank --->|
that seems to be the case. When clicking on 'Smart Dimension'
or on 'Distance Between' (without selecting any dimension)
look into the Ribbon Bar. There is an icon with an X on it,
click to open the Pull-Down and click on the 'X' in the
upper left corner.