I had a Lyman 45 cal Postell that did that to the point of me wanting to launch it into oblivion. I fixed it this way. First, I cast a good bullet from it in WW material. Then I drilled a 1/8" hole dead center in the bullet and tapped it for a screw. I drove in the screw and ground the head off. Then I chucked the whole thing in my cordless drill. Using "fine" grade auto rubbing compound, I ran the bullet in the mould for a couple of hundred revs. Then I recoated it and ran it in the opposite direction for the same number. I cleaned the mould very carefully in hot water and detergent, scrubbing it with a fingernail brush. After it dried, I took a single edged razor blade and a jewler's lupe and using the lupe for a close up look, scraped the vent lines clean with the blade. I was careful to pull away from the cavity so as not to create a burr of any kind. Then I carefull scraped the edges of the cavity with my fingernail. If I felt sosmething catch, I used the blade to gently scrape it away. Finally, I coated the interior of the mould with Rapine mould release. Now bullets drop from the mould as soon as I open it. According to my before and after bullet measurements, the dia. as cast did not change by any measurable amount.