you may have over tightened the bolt,
the brakes bushing can be made thinner,
or the boss made longer by filing the shoulder back, without
removing any of the smaller part the brake rotates around
a file with no teeth on the edge will be key for that..
or you can clean up the threads to be grease free, inside the frame boss
and the bolt, apply loc-tite, and rely on its bonding to hold the bolt in
without overtightening it.