Has anyone else ever had this problem? At some points in my drawing I can pick points to dimension to like where two lines meet or a line meets a radius. But some others I cant? Why is this?

For example, above I am able to pick that point.

But the pic below, I am unable to pick that point. Its like this throughout the whole drawing where some you can and some you cant. Definitely more that you cant though. I cant see a pattern. What am I doing wrong?