If I were you I would drop Jason an email explaining the problem. He has helped me out in the past and seemed only to keen to help. It might be that you have a faulty batch of moulds, or he might be able to give you some advice, but he is certainly a nice guy and would iI think would be mortified to think his product is not working as it should.
I have the 5 3/4 oz flatback that takes the clip downs, and have no problems with it other than the one time I forgot to put the grip wire guides in and filled the holes with molten lead, that was fun trying to clear them out, did teach me a lesson though.
Other daft thing I did was trying to cast 3 or 4 leads at a time (using my other moulds) and poured into the namix, but forgot to pull the guides out while the lead was still soft, again bu88ered a set of wire guides up, another lesson learned.
Do you pre heat the mould before using it as this also helps?
Good luck hope you get it sorted.