If you are talking about the report under Reports|Transaction Reports|Sales, then you probably didn't say yes to the question "Do you want to clear the sales file?" when you ran the report at the end of February.

Although you didn't ask for such advice, OrderEntry is not the best database to use for Sales Reports, although there are a couple of rudimentary reports. The problem with OrderEntry is that it is too easy too delete history, and as you found out it is too easy to miss the cutoff. The best place to go is Adagio SalesAnalysis. If you don't own that module, you should take a look at it.