Are you using a braid or manifold or .... ?
I have had similar challenges (low 70s) that I suspect are due in part to channeling issues / poor manifold design.
More aggressive stirring and longer mash times have helped a bit.
A friend of mine just made a prototype High Density Polyethylene (HDPE) false bottom for my 60qt rectangular cooler that I'm going to try next weekend. Hoping that has some positive impact as well.