Trying to perfectly snap align a line, doesn't work.

freshx,

There may be different reasons, inculding a corrupted document or a general corruption, but:

As I (mis)understand it, you wish to move the top line down to coincide with the bottom line, and to have the other objects retain their positions.

In this case, a crude way could be to:

1) Create a nostroke, no fill rectangle with the same width as the lines and a sufficient height to reach beyond the other objects, then place it with its centre on top of the centre of the top line; you can simply use the Transform palette with the centre Reference Point selected and give it the same X and Y values;

2) Group the rectangle with the top line and the other objects;

3) Give the Group from 2) the same X and Y values as the bottom line.

You can delete the rectangle afterwards, or just leave it for possible later use.

Do you have Smart Guides turned on?