Sometimes found is the hole in the frame the crossbolt fits into is mis-located by a bit and the bolt can't properly lock the cylinder arbor in place. Study the fit, you might remove material from the groove in the cylinder pin, or from the crossbolt. Check for burrs. As others suggest, a stronger spring may solve it.
If it is the "newer" Uberti with the long, double grooved cylinder pin safety device, the hammer striking the rear of the cylinder pin in the safety position can damage the crossbolt.