They didn't say much but I believe it was a combination of...

They didn't say much but I believe it was a combination of the grades being much lower then expected (in part due to dilution) and the mills recovery rate dropping off on low grades.
It was a very cheap gravity mill. I believe it wasn't able to effectively recover much gold from sub 5 gram dirt which I think was the cutoff grade.
So in theory if you put through 100 tons of 3 gram dirt you should get maybe 250-275grams of gold but in this case they we're getting absolutely embarrassing amounts amounts like say 80 grams.