Ah, ok, CSTI is a special case. I think they are just plain incompetent, or their systems suck. Most of the times I've had to complain about an order not being displayed, it has been when the trade was routed to CSTI. They also take forever to confirm cancellations.
Another thing that I just thought of is that in a fast-moving stock at the open, orders often get held for a few critical minutes. I'll concede that there's something fishy about that, but it's not what we've been discussing with IMSC.