I think your problem is that you froze the bullets & they shrunk up just enough to permit the lube to pull away from the bullet. When you froze the bullets & the lube was the lube brittle?
When I pan lube, I try to have slightly over one bullet dia. between the bullets. This will preclude the lube from pulling away from the bullet or cracking.
I also pre-heat the bullets w/ a hair dryer prior to pouring in the lube. I let the bullets become warm to the touch, then pour the lube. A warm bullet, like a warm mould, I believe allows the melt to slowly set up in the grease grooves & adhere better.
After pushing out the bullets, all the grease grooves are filled out.
When re-melt the lube for another pour, I use a double boiler to re-melt the lube. Most lube dont tolerate well the heat of direct flame
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