This is called "Cylinder roll-back".
In most cases its a worn or over-beveled locking bolt.
In an older gun it could be a combination of a worn bolt and one chamber locking notch worn or damaged.
The "fix" ranges from elevating the bolt, to replacing the bolt, to replacing the entire cylinder assembly.
First step is to check the cylinder for damage or wear in the locking notches.
If the cylinder is damaged or worn, this requires cylinder replacement.
Cylinder replacement requires some tools and a very high level of skill and experience at fitting and adjusting a cylinder assembly.,
You usually also need a like-new ejector that still has enough height to allow refitting.
If the cylinder is usable, you may be able to elevate the bolt if it isn't too bad. This requires readjusting the entire action.
If needed, the bolt may need replacing.
New bolts are hard to find, and are one of the Colt revolver gunsmiths major jobs.
This requires a LOT of knowledge and skill since the bolt is one of the most complicated gun parts fitting jobs there is. These parts are not even close to "drop in" parts.
I think Jack First sells newly made non-Colt "replica", bolts and these require some alterations before they can be stated on fitting.
If you want to tackle any of this, be smart and buy the Jerry Kuhnhausen shop manual.
This was written as a training aid for new gunsmiths and goes into great detail on just these problems and how to fit new parts. Most important, it shows how to diagnose just what's wrong and how to judge what needs to be done to correct it.
http://www.brownells.com/.aspx/pid=25720/Product/THE_COLT_DOUBLE_ACTION_REVOLVERS___A_SHOP_MANUAL