First step with a new Lee mold: check alignment and deburr.
Second step: face the top of the mated blocks and the sprue plate with emery cloth on a piece of glass.
Third: cast with it, so you can figure out what else is wrong.
I haven't lapped one yet, but I do have a mold that would probably benefit from it.
"Such is the strange way that man works -- first he virtually destroys a species and then does everything in his power to restore it."