Freeze Plug blowing out

I've never once had trouble with silicone on a frost plug....................uh excuse me, CORE plugs

How are you driving them in? If you are driving INSIDE the cup rather than the edges, this might tend to pull the sides inward

Now in the '70's I had a machine shop go through a 440 block and they CLAIMED that one? plug hole was OVERSIZED, but that is a distant memory, so, don'know............