I don't have a Pietta, mine are Armi san Paolo,
and Armi san Marco. But, a lot of makes have a tendacy to start binding after a few cylinders are fired. The best suggestion is to try several different lubes on the cylinder pin and in the chambers either on a wad under the ball or in the cylinder above the ball. Some swear by Crisco. I use my own concoction of beeswax and tallow with just a little crisco to make it softer, It's still a lot stiffer than straight Crisco. Some like the White Lithium grease for lubing the pin and loading lever assembly. I also just resolve to wipe down with a damp cloth and re-lube after firing 2-3 cylinders. Just gun oil on the pin won't last long.
Jon D