When I am pulling geometry off an imported solid model sometimes I have a surface or a line that will not read a radius with the dimension tool.
In a similar deal - sometimes I can not pick the mid point of profile curves - in the model or even after I bring them into a drawing.
I have got around it buy making another sketch and setting points eye ball close to the mid point - coincident to the line and then dimensioning to an end point until it is centered - with a missing radius I draw a circle - add coincident constraints to the end points, and change the circle diameter till I get close for a size.
When is a radius not a visible radius....?
What geometry lets you pick a midpoint & what will not let you...?