Can't figure this one out - zoom affects my lines!

[First Draught]

Hello all,
I'm pretty new to this (done some BASIC and C+ before but never even heard of LISP till last week!) so not sure if this is a common problem or just something a bit odd!

I've written a little procedure for drawing T-Bars which I thought I had working properly. Was having some issues with OSNAP but then figured out how to turn it off and then set back to it's original values. Now I've found out something which has been doing my head in and can't find anything related in any searches:

It draws the Tbar just fine (dotted line in the centre and two solid ones on a seperate layer, one at either side) then if I zoom out a little and draw another it misses one of the solid lines, zoom out further still and it draws the two solid ones but not the dotted one (or more accurately one of the solid lines is on the dotted one)?!?!

The thing I find most odd is I modified it to draw from profile and no matter how I zoom etc can't get that one to do it!!

Thanks in advance for any help

Was obvious really, had missed out one line (but kinda read it as I thought it was)