SG lower when building vs. downloading recipe

I'm trying to design a beer by building one ingredient at a time.  However, my estimated SG seems low (very low).  Even when I build a recipe that's identical to one I download, the estimated SG of my built recipe is very low (1.012-1.013) vs. the downloaded (1.035+).  What could be causing the difference?

when you download the recipe, are you using the scale recipe feature to adjust the recipe to your profiles?  That would be the first thing I would look at.  The person who posted the recipe will have the recipe adjusted for their brewing process and results.  YMMV.

Thanks for responding. I thought of that.  I generally scale to my equipment.  (I have fermenters for 1.5, 2, and 5 gallons and a 4 gal brew pot).  For my 5 gal setup, the 'scratch' recipe was calling for ridiculous grain bills, like 20-30 lbs of base malt to get close to the target OG.  Depending on style, of course, my scaled grain bills from downloaded recipes are in the 5-10 lb range.

have you checked your mash set ups?  what type of brewing are you doing?