I lost $2000 once on a nonexecuted hard stop on ES futures at IB once. The price went to my stop, IBKR did not execute. The price violently went threw it and I had to close it manually an hour later. I was not in front of the screen at the time.
IB has so many flags that controls your order flow and by default they won't kick your stop outside RTH (regular trading hours). But buy default they'll execute your buy.. I had similar issue in DEMO account during my initial days. Did you play with their DEMO a/c for at-least a week before going live? If not, blame yourself first.
It doesn't matter. Scooby is right. What you wrote about a stop order's position in the queue dictating whether it gets triggered or not is utter horseshit.