if washers won't give it change, you can make a shim out of plywood, but it's a bit more work as you have to sand the angle into the ply which should be the same dimensions as the back of the mount overall.................what kind of mount is it?
Great Planes 2 piece mount will allow the use of washers, but i have seen a couple of 2 piecers that would not shim out with washers.............