I made a brew today and I have been racking my brains trying to work out why my O.G. was only 1.020 at 20 degrees c.

This is a recipe I had made before in the States using LME.
Previously I had used 6 Lbs of LME in a 5 US Gallon brew (19L) and got an O.G of 1.042.
This time around I have been using UK sourced Ritchie DME (light and medium). I used 5 lbs for the same 5 US gallons.
This means I am only getting about 20 points per US gallon yield (sorry about the US style measurements but it is where I got started) where I would expect between 36 and 40.
Is there a difference in yield between US and UK LME/DME? Does anyone know the yield for Ritchie's DME.
I am really annoyed as I think I have scuppered my first brew of 2009, with a very lethargic fermentation so far.
Any advice on good sources of DME would be great as I have found choice very limited compared to the US.
thanks for any advice offered. Matt