The part of the barrel that leads can tell you alot about what needs to be changed. The following is based on my expierence and like always YMMV.
*Leading towards the end of the barrel tells me that the lube is failing. I will either need more lube, or a harder lube to last the trip down the barrel.
*Leading down the entire length of the barrel usually means that the bullet is undersized, or the pressure is too much for the alloy.
* Leading at the throat can mean an undersized bullet or A bullet that is too hard for the pressure ( it doesent expand to make a seal)
I have noticed when pushing a cast bullet really fast that somtimes I will get leading on only one of the barrel's lands. After researching and testing I concluded that the land that gets leaded is the one that does most of the work, and the velocity is too high.
Alot of times seating a cast bullet will swage down the bullet. Pull a seated bullet and then measure. The lee FCD takes away any advantage of having an oversized bullet. I load a lot of cast with bullseye in 9mm and 40 s&w, with very little to no leading. I also like HP38 for cast.
If you can provide where the leading is at and your loading routine maybe I can help a little more.