its hard to miss OG on an extract kit unless you added way too much water. like its a 3gal recipe and you used 5gal.
but we'll assume you didn't do that repeated times.
What temperature are you taking readings at? 60F is what 95% of hydrometers are calibrated to, and if you're trying to take a reading at 160F...forget about it. it gets way too skewed by high temps.
another VERY likely possibility: are you doing full boils, or smaller boils and then topping off the water in the fermenter? I ask because its REALLY common for the fermenter to not be very well mixed, even if you stir the crap out of it...that thick wort just doesn't easily meld with pure water. the bottom of the bucket might be 1.080 cuz the wort has settled, while the top where your hydro reading gets taken is 1.040.
it stratefies in the fermenter...if you use a glass carboy you might even see how much lighter the top half is than the bottom.
I had low OG's in my extracts too, but tasting and drinking the final beer, you could tell it had the correct ABV and not a full 1.4% lower than my OG/FG calculations yielded. So I had to assume I did hit my OG, and did have the 4.4% alcohol since the beer tasted 'right' and had the appropriate buzz after a couple pints.
forgot to mention that old LME cans can have sugar 'degradation' that can screw up OG and fermentable sugar levels.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10