Hmmm, others may have a better idea but maybe your gibs are bent (Or just never were flat to start with)?
I took mine out and flattened then using sandpaper glued to a granite tile.
This page shows the proceedure...
If the dovetails are OK, with a flat gib you should be able to adjust it so the play is about the same through the whole travel.
You mentioned adjusting it... there are 3 adjustment points, are you using all of them?
This page walks through adjusting gibs...