1) Which mould are you using?
2) Is your mould one that forms Boolits designed to be used with a Gas Check, if so are you using a gas check?
3) Have you slugged your barrel to get the true diameter of it?
If you are getting leading in the barrel, there can be a number of causes, all of which are simple to fix. It may be easy to clean out now but over time it will not be so easy to get out. If you haven't slugged your barrel i suggest you do so. Manufacturing specs are usually pretty spot on but some times can vary just enough that the boolit you're casting is too small. Because you're casting lead boolits, the general rule is to cast .002 larger than the diameter of the barrel. If the boolit is too small, leading will occur which is why you should up-cast and push through a bullet sizer a maximum of .002 larger than barrel diameter. Because lead is soft it will compress when fired creating the optimal fit and pressure, in turn preventing leading. Use a micrometer to confirm the proper cast. Never cast and use without verifying the first 20 or 30 boolits. Once comfortable with the mould I verify two or three out of every 10.
If you're not using gas checks on a boolit that came from a mould designed to be gas checked, gas will pass between the boolit and the barrel and / or melt the base of the boolit ever so slightly - but enough to cause leading. It may not be noticeable after a dozen rounds or so but put 30 or more through it and you will start to see buildup.
I cast hundreds of boolits and have zero leading issues in my .303 British Enfield. The barrel is .311 (or so it should be), was slugged at .3125, and I cast at .314 and and size for .314 with a gas check. No leading issues. Castboolits is a great source for people who cast. Lots of info - the site is dedicated to casting boolits after all.
Sawzall mentioned Lee recommends not re sizing tumble lubed boolits. I find the difference is between sized and non sized is irrelevant if the boolits have the proper amount of lube thrown in when tumbled after being sized. Lube the boolit before sizing. Put it through the sizer and gas check if required, then re lube. If its done properly with an even coating, then you shouldn't find any issues at all.