When brewing from an extract kit we always seem to be low in the Original Gravity department. It's usually not a lot -- something like 1.055 instead of 1.060. What might be causing this to happen? Is there anything we can do to check and adjust before the fermentation begins?
If you're doing a concentrated boil, it's possible you're just adding too much water to the fermenter when you finally put it together. For a five gallon batch, a difference of about half a gallon of water will cause that exact difference in OG.
If you're doing a full wort boil, it could be the difference between boiling with the lid on and boiling with the lid off.
The usual problem is that a brewer will do a partial boil and top up the fermenter with water. Since wort, containing sugar, is heavier than water it will sink to the bottom of the fermenter. When you take the sample, you're getting watered down wort from the top of the fermenter. When doing a partial boil, it's usually easier to compute the OG than to measure it.
If you've not adjusted for the temperature of the wort then this might explain a few points difference. Check what temperature you hydrometer is calibrated to (usually 20°C/68°F) and then use this calculator - http://www.brewersfriend.com/hydrometer-temp/ - A 40°C wort would explain a 1.060 gravity being read as 1.055
Another possibility is the kit is just overstating the potential OG. My kit suggested 1.042, but I have just put my ingredients list into http://www.brewersfriend.com/allgrain-ogfg/ and got an OG calculation that was very close to what I measured.