First, the gun only has a single
cylinder! Do you mean the cylinder will not rotate on one chamber? Colts are completely different than S&Ws mechanically so nothing that applies to a S&W works with a Colt. Is the mechanism clean and lubricated? Have you checked the ratchet for a damaged tooth that the hand may not be engaging properly? Is the bolt (coltese for cylinder stop) retracting when the hammer is cocked, or is it still up and locking the cylinder?
Protocol Design is completely correct, DO NOT mess with trying to modify parts to correct a problem in a Colt revolver. Parts fitting in a Colt is extremely critical, and a very tiny change in one part can affect everything! Fitting of the bolt is one of the most critical portions of the Colt mechanism. There are very few gunsmiths capable of working on Colt revolvers anymore, but trying to diagnose a problem on these revolvers without being hands-on with them is futile, especially for anyone who has to ask this sort of question on an internet forum!
Other than taking the sideplate off cleaning and lubricating the action, and closely observing how the parts are interacting. I cannot give you any better advice at long range!
Finally, you cannot check cylinder timing with a range rod! They are only intended to check for
gross mis-alignment that would result in bullet shaving and possible injury to surrounding people.