loose cork stopper

Perhaps start with a tighter fitting cork. They are available in a variety of diameters.

Boiling a cork may restore its diameter a little.

Some technicians put paraffin wax over the cork, position the cork in the head, then heat the head to melt the wax, presumably to seal the cork in the tube.

I don't think many technicians would attempt to re-use a loose cork. A well-sealing cork (and that means at the embouchure hole end of the cork) is very important for a flute.

daigle65 said:

I spoke to my flute tech today and he said to soak it in hot water for 5 minutes as a temporary measure, he will replace the cork. He was surprised and so was I, I've been playing flute for 25 years and this is the first time this has happened.

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I boiled a batch of undersized corks which I bought. They turned oval. MORE likely to leak, because the smaller 'diameter' was less than the metal end of most crown cork assemblies.