Flashing on large castings

Hi

I am having some problems with my castings that are over 150 dwt when
I am melting I see that the silver is cooling in the crucible and
not getting a uniform melt. I am using a victor #3 torch tip with
acetylene and oxygen. The problem I am getting as I have been told is
called flashing. The small flasks are casting perfectly and the big
and the small flasks are invested at the same time using the same
equation. I have just started casting since last month and usually
only cast a small quantity. I have yet to cast a big flask with
success.

Thank you
Gary Udhwani