With extract brewing, there are really only 4 things that can affect the OG:
1. The wort is not well enough mixed, and the sample is slightly less dense than the wort at the bottom of the fermenter. This is nothing to worry about.
2. There's something wrong with your specific gravity measurements. Either the hydrometer is not calibrated or you are not properly compensating for temperature.
3. There's not enough malt extract. Either the kit is slightly under weight, or there's some waste (sticking to the sides of the container, perhaps).
4. You add too much water. The O.G. is based on a given total starting
Regardless, the goal is not to hit an O.G., it is to make good beer.