At risk of turning this into a reloading thread....
9mm is a high pressure round and because of this, things that may apply to .38's or .45 Colt or even .45 ACP may not apply fully to the 9mm. The biggest mechanical issue with hardcast lead in a 9mm is leading but the question is what is the main cause.
Well, gas skipping is most often brought up as the reason for leading in a barrel. Gas skipping is the action of gas from a fired load that skips past the bullet as it goes down the barrel. The gas is traveling faster than the bullet is and this is what pushes the bullet down the barrel (I know, Duh!) but if the gas can find a point to get past the bullet, it of course will and with hardcast bullets it will strip lead from the sides of the bullet and deposit this stripped lead onto the barrel surface.
Anyone that has cleaned an extremely lead fouled barrel will tell you it is a giant task and may just say screw it; no more cast lead bullets in this gun.
Now what caused all this? Good question Cheesewhiz, you are a fine student and damn good looking too.
The most likely cause is the bullet isn't obturating well and filling the grooves in the rifling, allowing this gas skipping. Ferguson pointed at this as reason to go to .357" diameter bullets for his 9mm loads, this larger diameter should fill the rifling and let the bullet to obturate and keep the gas from skipping past it as it travels down the barrel.
The problem is that this doesn't work so well with 9mm, remember it is a high pressure round, that extra pressure will find a way past the bullet.
...and most reloading manuals warn against using larger than .356 bullets as this could cause pressure issues in the chamber besides possible fit and feeding issues in a tight chamber like CZ's or Sig's.
I believe the best thing to do is to look at everything in your loads that have caused this issue.
I think the biggest "pointed at" reason is the cast lead bullet's hardness, a harder lead will not obturate as well as a soft lead and with the higher pressure of the 9mm the gas finds a way. I also don't ever think you can stop the gas skipping 100% even with a softer lead but it will help. Using a slower powder will also help, Bullseye is a fast powder. It burns hotter than a slower powder and will produce more initial gas pressure when it ignites. Look at your load length, setting the bullet deep into the case will not only cause higher pressures, it will also leave the bullet shorter from the rifling and allowing gas to slip past before the bullet has even engaged the rifling. This may be the biggest issue here but I'm no expert, I just know that 9mm has to be looked at a little differently than some other rounds.
So let me go through my points:
1. Try to use a softer lead.
2. A slower powder will help.
3. Check your overall length, adjust them out so they are just short of engaging the rifling.
....Point 3. will take some playing around as you will need to find a length that will feed, not exceed 1.169" and the bullet does not contact the rifling when chambered. It needs to be off the transition by about .010" to be on the safe side.