I have been using Datek (now Ameritrade) for years for swing trading. I really never mentioned it around here (not PC you know)....ET being a more ES trading environment using IB. I actually like Datek/Ameritrade, and have had no problems ever with many hundreds of trades. The fills are actually really good, and market orders are filled about as fast as I can blink and refresh the screen to check. I also short stocks, and they have a good supply, very rare I get denied. I too was "switched" over to their Ameritrade screen, just a little different than the Datek setup. I think it might be you are canceling too close to the end of the session and Ameritrade does not get a confirmed cancel back from the exchange? I usually enter positions no closer than 10-15 minutes before close just in case I have a computer problem or connection problem. But I am just that "sort" of person, I could probably cut it closer. I too would be interested in knowing if Ameritrade sells its order flow and what sort of disadvantage that would be.
One thing I have noticed is that "day only" stop orders that are open at the close are stuck open for a good 20-30 minutes after the close. I've never had one actually triggered and filled, but I can't cancel it and trade afterhours until it finally expires. Ameritrade, like most discount brokers, sells its order flow. The routing disclosure document should be available on their website and tell who they sell it to and how much they get paid for it.