Welcome to this forum! There are several reasons why you are getting a lower reading than expected:
1. Too much water. If you are using the plastic buckets with gallon markings on the side, they are known to be inaccurate. It is best to measure and mark the buckets before starting your first batch. If you are using a carboy or better bottles, those need to be measured and marked as well.
2. Not mixing the water and wort completely before taking the reading.
3. Bubbles from aeration may stick to the hydrometer and give it boyancy, causing a lower reading. You should spin the hydrometer and allow the wort to settle out a bit (no bubbles) to get the most accurate reading.
4. Temperature. Specific Gravity will vary with temperature, but the adjustments are minimal. Still, you should take the temperature of the wort at the same time as the hydrometer reading and then adjust accordingly. I use the simple calculator here:
http://www.rooftopbrew.net/abv.php
5. Hydrometer not calibrated. You should check to make sure your hydrometer reads 1.000 using distilled water at 60 degrees (I believe that is the right temp). Adjust accordingly.
It could be a combination of several of these. The good news is that it may be closer to 1.051 than you think; the better news is that you will have BEER, delicious, and if a bit lighter then expected STILL GREAT!
I know others here have gone in and added more fermentables after pitching. Do a search on how much and how.
Main point is enjoy this hobby!
