Some revolvers were not dead-on accurate in the milling for the safety notches.
On yours is it consistently off the same amount for all?
If so you can trim base of hammer a little or open the notches a little or both.
When you turn it, does it then lock into place?
If not you may have to do a little filing to let hammer drop a bit further.
You may want to drop a wooden dowel down the barrel into each chamber
as you successively bring it to full cock.
Verify the barrel and chambers are properly lining up.
If they aren't, the timing is off. Could be factory or a replacement cylinder or frame.