It would be better if you could post your code, but I still reckon it's to do with those evil twips!

I'm guessing the problem is down to something farcical like...

Each pixel is about 15 twips and the screen can only display pixels so twips are rounded (sort of). Ergo, if something is 14 twips in then it is 0 pixels in. If it's 15 twips in, it's 1 pixel in.

try the stuff below which I've just invented now off the top of my head.

Code:
Line2.Y1 = (int((Line1.Y1 + Shape2.Height)/screen.twipsperpixely)*screen.twipsperpixely)+1
Line2.y2=line2.y1
If that fails (which is quite likely) then please post your project for us to have a tinker.