Tarawa - first of all, you need pure lead - or a slightly smaller ball, say .710".
Since the mould is casting oblong balls across the vavity, not on the seam, the reason is due to a buildup of lead on the closing surfaces - probably close to the entry hole which holds the blocks slightly open & casts a larger ball across the cavities.

The mould blocks have to be cleaned of all lead, then treated to a moly spray (Hoppe's or MS Moly) or Rapine's mould prep to help prevent lead from sticking to the blocks. Merely smoking the blocks is a quick fix that doesn't last as well as the moly or mould prep.

Also, if you are running your lead heat a bit high, the blocks are prone to picking up lead by a weak soldering (sticking) of the metal.

Lead shot of today is too hard for round balls - except for someone who has some experience with shooting muzzleloaders & knows how to load hard balls.(ball size and patch requirements)

This is a wonderful site, but for your specific problems, you should go to www.americanlongrifles.com . There are many people there who have a great deal of experience with muzzleloading.

Was just thinking, that if the ball measures .725" on the round, the bore must be even larger than that if the ball will enter so with the grooves diameter even larger than THAT, it must be a true 12 bore(.729") or larger yet. The .710" is the proper sized ball for you for a .020" patch - or thicker. Dop not use a patch that allows you to 2 -finger the load down the bore - it will be innacurate past 25 yards as well, the ball will slide forward in one barrel as you fire the other creating a dangerous situation to you and the gun, ie" ringing or blowing up. Do NOT use slugs for the very same reaspn. It is a round ball gun.

Your inability to seat the ball is probably due to too sharp a crown on the muzzles. You need to re-crown the muzzles and to do this, 320 grit emery cloth and your thumb are all that's needed. The use of a tapered grinding stone first, applied with an electric drill to get the angles cut will speed the process, but it can be done with your thumb all by itself. Place the emery over the muzzle stick your thumb into it and rotate while pushing down HARD. Some lube, like WD40 is fine. I shove a wad of cloth down the barret to catch the 'grindings' and stone particals, which comes out when you pull the cloth with the barrel 'worm'.
The picture below is a crown that allows loading a ball that is larger than the muzzle, with a patch that is .020" thick.

My own .14 bore rifle uses a ball that is .006" smaller than the bore, wiht a .027" denim patch and loads easly.

The 'starter' has to be of large size and have some weight. One pounding motion smack of the bottom of the starter with thw shaft held in your hand, is all that's needed to seat the ball and patch flush with the muzzle. Turn the starter around and with one smack of the hand on the top, the shaft puts the ball some 5" or so donw the bore. The rod is used to shove the ball down to the powder, in 5" or 6" shoves, choking up on the rod.

If you want to shoot accurately and all day without having to clean, this is how to load & use something like Hoppe's #9 PLUS for lube. Regular Hoppe's #9 is for modern rifles & smokeless powder, #9 PLUS is for black powder. If you just want to make smoke and have go clean often & get miserable accuracy, use a thinner patch, with bore butter (it's merely lip balm with wintegreen oil to make it smell good enough to eat - really) and it's a lousey lube, sticks & builds up in the corners of the grooves and causes innacuracy after just a few shots.

Here is a crown that allows loading combinations that give good to excellent accuracy.

Due to time involved preparing this responce, I'll edit to include the picture.