With the gun unloaded, wrap a piece of masking tape around the back half of the cylinders outside making sure that the tape does not cover any of the small notches. This tape will be used to mark the chambers that have a problem.
Then, fully cock the hammer slowly. Then, without touching the cylinder slowly lower the hammer noting if it hits the side of the notch by the nipple. You may want to mark the chambers that the hammer interferes with.
Repeat this with each chamber.
After doing this, fully cock the hammer slowly and then try to rotate the cylinder with your hand to verify that the cylinder bolt (stop) has engaged the notch in the bottom of the cylinder.
Following this, slowly lower the hammer to the fired position.
Repeat this with all 6 chambers noting any that are not "locked up" by the cylinder bolt.
Repeat this several times.
If the cylinder bolt is engaging the cylinder notch and locking up the cylinder for each chamber (it won't rotate by hand) and the hammer is hitting the side of one or more of the slots by the nipples on some of the chambers it indicates the lock bolt notch's were machined incorrectly.
About the only "fix" for this that I know of is to file away the offending material from the nipple slot and to reblue with a Cold Bluing.
If the cylinder bolt is not engaging the cylinder notch at one or more of the chambers and rotating the cylinder by hand until the cylinder bolt engages the notches and, when the locked up cylinder does NOT have a problem with the hammer hitting the slot for the nipple, the "hand" is probably worn and should be replaced.
I've seen quite a few Colts that wouldn't lock up the cylinder when they were cocked slowly but cocking the hammer fast seemed to fix the problem. That is, when fast cocking the hammer rotated the cylinder rapidly the cylinder would rotate on its own far enough for the cylinder bolt to engage the notch's without help from the shooter.
If this "rapid cocking" seems to fix the problem you can make a mental note to yourself and just use that method of cocking the gun rather than replacing the "hand".