A couple of things that can come into play are the size of the chamber at the very back, and the length of the chamber vs. the length of the die.
If a reamer is too small at the very back of the chamber it can cause a problem as the web is increased in diameter by repeated firing with hot loads.
Imagine a hamburger patty (raw) being pressed between the opposing palms of your hands. It gets bigger and thinner.
The easy way to tell that this is happening is to use a .0001 mic. To measure the diameter of the rim of a case before firing, mark the case, and record the measurement. Fire it hot 15 times or so, and then remeasure, in several places. You will see that the rim has grown in diameter, and that the diameter is less even than at first. This is all predicated on loads that are near or at the “upper window” for a 6PPC.
Another situation can occur when the length of the body portion of the chamber is sligltly longer than that of the die. An overly large chamfer at the mouth of the die can cause this. If you have a case that clicks, put a bullet in the fired case neck (to keep it from collapsing), chuck the neck in a cordless drill (lightly), support the back of the case against a piece of smooth wood, and use some sort of fine abrasive on a stick ( I used a coarse diamond lap) held parallel to the centerline of the case to reduce the diameter of the case in the area immediately bordering the extractor groove 2-3 thousandths, and then polish with 0000. Load and fire the case as hot as would have previously have caused a click. Where did it go? You just "blueprinted" the leading edge of the web back to new brass diameter. I'll bet that you didn't get the click on the first full load firing of a new case, unless you have a defective action, or chamber.
In my experience regular dies don’t do much about reducing the diameter of the actual solid web of the case. They may reduce the case immediately in front of the web enough to ease bolt closure, but with an enlarged web immediately adjacent, firing springs the case into interference. If the web is smaller, the case has a tendency to pull back from chamber diameter after firing.