I did some quick crunching. It looks like you were only expecting about 51% yield, which is extremely low in itself. A very conservative 70% yield -- easily achieved with a conventional (I hate the term "fly") sparge and your ratios -- should get you an OG north of 1.067. I would guess there are two issues. First, at your water-to-grist ratio of 2.6:1 (1.23 qt/lb) it may have been difficult to adequately stir a 10 gal, 20 lb batch. (5 gal, no problem.) Could have left dry spots, even. Then there's the speed of your sparge. I'd say you should try a thinner mash -- at least 3:1 or 1.45 qt/lb -- and slow down the sparge. The proper rate for sparging is really dependant on the depth of the grain bed, not the total weight or volume. If your depth is on the order of a foot, recirculation and runoff should take probably an hour or so. Really, let the rate set itself: find where it flows smoothly and crystal clear by gravity without pulling solids through the filter bed. Thoroughly mixing the mash, periodically stirring it to help sugar migrate into solution, and letting the sparge percolate through more slowly should get much more extract out.
(Fixed typo)