I started with sketchup a couple of weeks ago, and also had similar problems.
The program is good, but I feel it sometimes oversimplifies things :(
I sometimes correct this kind of things by using "guide lines" and tracing the line to the exact points of the guides. But I haven't been able to correct a line if it is attached to other lines.
Rubén Andrés Martínez A.
(I love the smell of paper in the morning. )