AC2008 & Dynamic Blocks: Can't get stretch to work

I've been trying to solve this problem for hours, using tutorials from
autocad site, to no avail.

What I'm trying to accomplish is very simple. I have made a block that's
a single horizontal line. I've assigned a parameter point at the bottom
end of the line, and a stretch action so that when the parameter point
moves, the line will stretch along the point. However, IT ALWAYS MOVES.
It never stretches. I can't figure out what I'm missing.

 

Wrong to assign a point

1) is preferable for your line to have one extreme point as insert
point
2) edit the block and use a linear parameter: it creates two arrows
3) delete the cyan arrow on the insert point (0,0 coord of the block)
so your line has one fixed point and one mobile point marked with an
arrow
4) use stretch on the linear parameter and select the point with the
arrow as stretch area

Now it should work
For instance, to stretch a single line, the grip method is the best