I would pull a small sample (like a pint) and add a metric ass ton of yeast to it and see how it attenuates. That will let you know if it is a problem with the mash or the yeast. You need to figure that out before you can begin to address the problem.
How many packets of yeast are there in a Metric Ass-ton? I need to incorportate that measurement into my system. My next R.I.S might just nees a Metric Ass-ton of yeast to ferment properly!