I was trading AMZN today, when I went to enter a small short sell 'at market' order. I have plenty of funds in my account. There were numerous bids on their 'active trader' Level II view, so there wasn't a lack of bids to buy my sell at market. It didn't trade. It just moved up and down, tracking the highest bid. Meanwhile the stock kept moving lower and lower. I watched as my potential profits evaporated and my risk increased. It was all happening rather rapidly. I closed the short sell order. I have had this happen a few times recently, and with all the problems I've had with the TOS software, I assumed this was a problem with the software. But it's happened enough I decided to contact TOS support on this. They informed me that, beginning in mid-August, with the change to a different clearing house (I think this went from Penson to TDAmeritrade, but not positive), they now have a "team of people" that manually review and and clears short trades! This isn't an assumption on my part- this is what TOS support told me! They also told me to enter stop orders and that "would help". However, with the market turning and running out from under me, I wanted an "at the market" short sell. I don't even know if I could have gotten a stop in, as it would have probably ended up above the market. This inability to place the trade resulted in a significant loss of profits to me today. Note that this wasn't a lack of stock to borrow, nor a lack of funds in my account, otherwise a stop-sell wouldn't have helped. It is the time required for a "team of people" to manually review my order before it is "cleared" to be enabled for transacting. Support told me they are "in discussions" about this, but that it would not be changed anytime soon. They also acknowledged that this wasn't mentioned in any of the information provided to me when they announced their change of clearing houses. It was just left up to me to find out right when I needed it. So I'm now going to have to change companies, and learn a new platform. If you trade short, note this was a problem for me. If I hadn't seen it, and asked about it, and been told by TOS support, I wouldn't have believed that they were manually clearing short sell orders in a live market with a "team of people".