In order to effectively "tune" a fixed blade you have to be able to heat and move the insert
Oh no I have been doing it wrong all these years
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I have never had to heat and move an insert.
By the sound of it when your arrows where cut they was not cut square.
before you glue the insert in try screwing your Broadhead into the insert and then pushing it into the shaft and then SPIN test.
You can move the insert around untill it spins true then mark both arrow and insert apply glue push insert in so the marks line up.
If you cant get them to spin true then most likely the cut is not square.