trouble selecting edge of cylinder in sketch

Just a quick question (I hope).  I'm using SW2009, and having inconsistent problems selecting edges of cylinders in my sketches, either to add a mate or convert entity.  Sometimes I'm able to, sometimes not.  This morning I found that I was able to select an edge if it was the first thing I did in the sketch, but not if I drew other lines first.  I also found that once I selected one side of a cylinder, I wasn't able to select the other side.  Very strange, and I'm just wondering what I'm doing wrong.

Thanks for your help!

Michael

SolidworksSketching