A few reasons why this can happen -
1. Sometimes the change is too small for AC to pick up. It's about 0.02mm and sometimes AC will snap back to the original position. Soln. Move the line away 500mm then back again.
2. You have grid snap turned on and it's snapping to the grid and not the line you want to.
3. The line isn't straight. Best to redraw the line square if it's too fine a change. Opt. Use guidelines or the drafting tools. Looks pretty straight. Try Soln. 1 or 2.