Two things come to mind. Both of which you addressed already and that is cleaning and heat. Even a little residual oil can cause wrinkles. The other possibility is that the mould isn't staying hot enough. That little bullet doesn't have the ability to heat the mould quicky. Try heating the mould longer in the melt. When I cast two moulds, one in 400+ grains and the other the same mould you have, I get bad bullets too. My secret was to heat pot more, keep mould hot, spend less time between casting's.