Unfortunately, that's not really enough information to speculate. Which broker are you using? Is there any pattern to the missed trades? Any error in the logging in RightEdge when the pending order is not created?
My live system seems to ocassionally ignore trade signals based on 1 minute data. In those instances, it does not generate pending orders (it goes without saying that the live system was activated). However, when I run a simulation these trades do show up in the trade list results as expected. What could have caused this?