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Those trades could be in dark pools or other matching mechanisms. While specifically answering your question, there are more aggressive order types such as such as peg to midpoint https://www.interactivebrokers.co.uk/en/index.php?f=38079 that you could try to be more aggressive to get to the top of the book. Also note, exchanges could also have order types or algos such as ISE with there midpoint match https://www.interactivebrokers.com/en/trading/pdfhighlights/PDF-MidpointMatch.php that could explain some things.