I did a quick search and found this on the High Road forum, it explains lock up; you can read the entire link at the bottom.
dfariswheelAugust 28, 2010, 09:11 PM
If you have an old style Colt like a Detective Special or Python, the cylinder should lock up tight only when the trigger is pulled and held back.
If you have a S&W, Ruger, Dan Wesson, Taurus, Rossi, or a newer Colt like a Mark III or King Cobra, the cylinder is intentionally designed NOT to lock up tight.
This allows the bullet passing from the chamber to the barrel to force the chamber into alignment with the bore.
In these types of guns, the action will lock up as tight as it's going to with the hammer just cocked.
Pulling the trigger won't lock it tighter.
in other words, if you have something other than a older style Colt your gun isn't broken and trying to "fix it" will prevent it from operating properly.
Cylinder lockup - THR