I have been looking into that sort of problem for 2 years. Best I can tell, is your bid needs to be for 5000 shares or more, with no restrictions at all except the bid price, other wise the MMs can ignore your bid and buy the cheaper shares for them selves at prices below your bid (especially for orders that are sell at market orders). If the ask gets down to your bid, with out a fill, it is possible older bids at your bid price were filled first. If the ask goes under your bid, with out a fill, it may be a low volume issue. If the ask goes below your unrestricted bid (where the only limit is the bid price) and the volume is there, then you have a gripe that your broker will investigate if your order was by passed and not filled.
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