me.line method only works sometimes

Access 2003

I am trying to use the Me.Line method to draw vertical lines, but it only 
works sometimes depending on starting point of the line and how tall the 
line is.

For example:

Me.Line (442, 0)-Step(0, 1440)   draws a vertical line
Me.Line (443, 0)-Step(0, 1440)   no line appears
Me.Line (443, 0)-Step(0, 635)    draws a vertical line
Me.Line (443, 0)-Step(0, 636)    no line appears
Me.Line (494, 0)-Step(0, 1440)   no line appears
Me.Line (495, 0)-Step(0, 1440)   draws a vertical line

I am trying to work out why it is behaving like this. Any suggestions?

Thanks,

Adrian