Are you properly lubing the mold pins, sprue plate, and all moving/turning parts? I've used a lot of Lee's 6 cavity molds and never had that problem. But I do keep things well lubed, usually lubing before I start every new casting session. Pay extra attention to keeping it perfectly closed while pouring the lead, otherwise lead may leak out onto the mold faces and prevent it from closing later. Is anything loose on it?
You can try giving your most problematic mold a thorough inspection, cleaning, adjustment, and lubing and see if it helps. Keep the lube away from the inside of the cavities of course... If none of that helps, perhaps you really are the unlucky owner of multiple defective molds and a call to Lee is in order.