Welcome to the forum. I'm at home now and don't have SW here, so I can't open your model, but I have a suggestion. Assuming that the lines are from two different sketches, you might try using the Convert Entities tool to create another line in the active drawing and set a relation between these two lines. If that doesn't work, and you haven't gotten if figured out, I will open your model when I get to work tomorrow morning and see if I can figure it out.
Very interesting observation!
The reason that you can not add relations between some lines ("entities") is because projection of those lines does not exist. You have to be very careful when you apply relation between entities across different sketches. Solidworks does not apply "EQUAL" relation in a normal sense, but rather "EQUAL" relation between projections of the lines. So, if project of a line is a point (no length), mathematically you can not apply this "EQUAL" or any other relations.
I hope this helps.
thanx, i just had the same problem and your explanation make sense. whats strange to me is this is the first time i've noticed it