How did you take your sample? If you drew it off of the top of your fermenter after you added cold water to bring up the volume and cool to pitching temps, it is likely that the wort was stratified, in other words, the thicker stuff was at the bottom making your sample read lighter than the actual gravity of the whole batch. Be sure to stir the wort well before taking measurments (with a sanitized spoon of course). Or perhaps even better, just dont worry about the hydrometer, as many suggest on this forum. If you included all the ingridents from the kit, and ended up with the right volume of wort, it has got to be close to what is should be (you were using an exctract kit, right?). Just check your recipe, make sure you put everything it it, and then forget about it till it's time to bottle.