Registration of lines as symbols?

Using Flash CS6

I am trying to animate some lines. Similar to what this guy is doing here: http://www.youtube.com/watch?v=3QoU1nFUSa0 at 29:25

basically 4 lines that come from left right top and bottom. Now I convert my lines to symbols, the line coming from the left I convert to movie clip and set its registration from the middle left. Adjusting the width then locks the point on the left and moves the point on the right. I can then add a classic tween to make it look like the line is coming from the left. Similarly, with the line coming from the right, convert to symbol, movie clip, right middle registration, motion tween, hey ho !

The line from the top is also fine, convert to symbol, movie clip, top registration, motion tween.

Now when I come to do the line that comes from the bottom, I'm assuming bottom middle registration here. When I adjust the height the top part of the line stays fixed and the bottom moves, just as when it was registered to the bottom in the previous line. However when I drag and move the height now, not only does the height decrease, but the whole lines moves further upwards.

What am I doing wrong? :confused:

When I make rectangles, the registration selected is correct, but not for this one type of line.

Cheers to anyone who can help

Also, If I make the lines slightly off centre, ( like this -----> / ) the registration works fine, top and bottom. But as soon as they are perpendicular ( like this -----> | ) they will only ever seem to be registered from the top.