As others have said, try mashing thicker to sparge more. Be sure the sparge is SLOW as well.
I had the opposite problem yesterday. Normally my mash efficiency on bigger beers is down near 65%, so I actually set BeerSmith to assume for this beer I made yesterday so that I wouldn't miss OG be 10 points. I hit 83% efficiency, and overshot pre-boil by 18 points. Guess this beer will be about 2% stronger than I wanted...
Now I have to figure out what the hell I did that got me normal strength efficiency on a beer with 17.5 lb of grain.