I noticed today that my buy orders for VXX vertical spreads (good-till cancelled orders that had been sitting for a while) did not get executed even if there were trades that occurred below my bid. In another case the ask went below my bid, and again no execution, and no trades in the market either. Is there any explanation for this? I would genuinely like to know, so that I can understand the execution or bid/ask data issues in spreads. I have portfolio margin and plenty of margin left (~75% of account NAV); however I have negative cash balance equivalent to ~10% of account NAV. Negative cash balance has never been an issue so far, so I don't believe that is the reason. I understand options are not marginable but that should not matter under portfolio margin. Thanks for any insight.