No real ideas without being able to see the gun.
The cylinder should push the cylinder release back and snap into the frame without pulling the release back manually.
There are a number of things that can allow the cylinder to hit on closing, most of which you see mostly on worn guns.
Unfortunately, none of these are things you can repair yourself.
Try giving the gun a good cleaning and some fresh lube.
If it looks dirty or gummed up, spray in a lube-cleaner like CLP Breakfree and allow to soak overnight.
If that doesn't help, your only real choice is to send it in to Colt.
DO NOT take it to any local gunsmiths. Only a tiny few have any idea of how the older Colt action guns work or how to repair them.
Often they will cause other problems that can ruin a gun or cost big money to correct.