You should be able to solder to brass - check with you flux supplier to get their recommendations. The flux you are using may not have enough activation to breakdown the oxides on the brass surface.
A word of caution - brass (copper and zinc alloy) can undergo 'dezincification' if there is no barrier plating between the brass and your solder joint. In some use environments this becomes a latent defect.
Your brass bits supplier should be familiar with this and have a standard process on his books. We'd expect to see a nickel or copper barrier.
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