Try drilling a couple holes in the threads of your plastic elbow below the FB. That's what I had to do to solve this problem.
Even with only water, the vacuum can sometimes be enough to pull that elbow onto the bottom and block your flow. With grain weight on top of it, it's almost guaranteed.
Technically your LHBS should be selling those elbows with the holes in them already. I know mine does.
Secondly, when I put in my water in to pre-heat my MLT, I tap the false bottom with my mash paddle and circulate a little bit of water through the valve to make sure that there is no air in there.