+1 on cleaning the cylinder.
The Python is a .357 Magnum, and I note that you mentioned shooting .38 Special through it. That's fine -- you're not the first person and you won't be the last to shoot .38 Special out of a .357 Magnum. But you have to remember that when you do this, the chamber will extend beyond the case for a short distance before stepping down to the bore diameter. This gap tends to build up an accumulation of hard, burned crud (for lack of a better term), consisting of burned powder residue, lubricant, and possibly lead and/or copper. If that carbon ring isn't periodically and carefully cleaned out, I would expect it to gradually migrate towards the rear of the chamber, and I can see how it might easily build up to such extent that it would begin grabbing the mouths of the fired cases.