Gary,
Very good question!
Often when fitting the bolsters to a knife, the bolsters may be worn or dinged so they may not be perfectly straight. You have to fit the scale to the bolster that is on the knife, not to what it should be. Many knives are alarmingly imprecise, even good quality knives. They were made as tools, not as precision instruments. So, be sure the bolsters are in fact straight before you begin fitting. If not, fit to the bolster on the knife.
I usually square my handles to fit the bolsters with the disc sander on one of my belt sanders. I have to be sure that the table and the sanding disc are truly perpendicular. That will work for bolsters that are perfectly square.
For most used knives, I actually undercut the bottom of the handle material by 3 to 5 degrees, even for a square bolster. The undercut is so slight on 1/8” thick scale that it is unnoticeable. It also leaves a bit of space in there for a good bead of glue to seal the space adequately and prevent moisture from penetrating under the scales.
Give it a try if you are having problems.
Let us know how it works for you.