Try adding tension to the bolt wings. Remove the bolt, clamp the forward portion in a vise, and with a screwdriver, gently pry the wings apart, biased toward the hammer side until they are not parallel. Careful not to break the ear off. This will increase the pressure against the hammer's cam and usually does the job.
If that is not enough, either the cam is worn, or the back edge of the bolt wing is too short, and drops off too soon.
While the bolt is out of the gun, you should make sure there is a small chamfer at the top cam-side wingtip, to help ease the bolt back up the cam in reset mode, a little job the Uberti assemblers almost never do.
Check a book, like Jerry Keunhausen's Manual on Colt Single Actions or other Colt SAA tuning book for the proper location and extent of that chamfer.
The lack of that chamfer contributes to the premature loss of spring pressure against the hammer due to over flexing.