Re-inserting drop glass. How do I get it in?

[flux3d]

My drivers side window glass has decided to fall out of the front runner while I was working on the doors. I've removed the glass from the window which went ok but I can't seem to get the window to go back in. I can get it to slide onto the front runner, and even part of the rear runner, but then the window glass gets jammed against the black strip running along the edge of the tollbooth window and won't slide down.

Am I missing something here? A search on the net just reveals people saying how easy it is to get them back on. Do I just need to be a little more forceful re-inserting the drop glass back into the window? I'm just scared that I'll break something.