Probably just a little too lean from the factory for emissions. If you only had a problem a few times, it could be that the temperature/humidity was a little different on that particular day. That can change it some. I notice my 5hp briggs on my db30 runs better some days than others if I don't adjust the carb. If you have a hard time starting it consistently, then you might have a problem. You could try drilling the jet out slightly or just find a carb that is adjustable, then you can change it whenever you want. If your going to do a lot of work with different carbs/jets, you can buy a numbered drill bit set with tiny bit specificly for jets. You can also buy them individualy if you know what one you need for a certain thing. Just remember that the number of the bits has nothing to do with the number of the jet, and different companys that make jets use different numbers for different sizes. In other words, just because the jet is a #59 for example, doesn't mean a #59 drill bit is the same size.
