I am also having this issue with my mac. I have been using since 7 and I have noticed a big difference from 7 and 8 to 2013 when trying to "pull tape". Its gotten worse, maybe? Could be user error, but I do use SketchUp every day and feel like its just become a problem. It seems to be just when you have a solid like your pentagon, grouped and you try to pull tape on top of it. I was able to find mid points along the side that ran along the axis but not off axis. Now…..If the pentagon was ungrouped or exploded at this point it did not seem to be a problem.

I also want to ask from the comment made before…. Why? does it matter in this case of infrencing which way you have you circle/polygon rotated. A midpoint is a mid point, Right? 10.8.5 > 2.66 GHz i7 > 8 GB RAM