A simple workaround is to move the endpoint beyond the snapping & sticky move tolerances, then move it back within the snapping tolerance of the other line. There is a snapping tolerance issue and a sticky move tolerance issue, (not to mention the feature topology tolerance) which is why zooming in is another solution, because you probably have your tolerances set in pixels. You could reduce the pixel size of your tolerances to change the snapping behavior at any particular scale. If you hit the T key you should see the snapping tolerance zone.
