Just brainstorm questions to rule things out and try to find the issue:
1. Was it 1.25 qts water to lbs grain?
2. What was your mash schedule? Single step, or multi step? What was the temp (that you had, not the recipe)?
3. Did you add anything besides the grain to the mash, such as DME or finings that should have been in the boil?
4. Did you add any adjuncts, like oats, that weren't in the recipe?
5. How many times during the mash did you stir it?
6. You checked the ball valve for clogs. Did you check the false bottom elbow for anything?
As a note, I use a manifold in my cooler, so I don't have a false bottom. But I thought the edges of the false bottoms were supposed to be pretty snug against the cooler. It looks like there's quite a gap between the side of the cooler and the false bottom. Maybe that allows the edges to lift up and get grain under them (again, I don't know that for sure).
