Check your plugs, MAF, etc. It's very odd to have a diesel so low - what kind of driving are you doing, mostly city? How do you drive - heavy foot?
Also, that dash max range is cumulative based on the overall avg MPGs. Average MPGs are calculated for your total combined driving since the last time you reset it - so the more you drive between resetting it, the slower it moves.
It uses the average to calculate the max, so if your average is low it'll think max is low. For example: if you do a ton of city driving and average is 30, then you hop on the highway for a few days you'll notice the "miles remaining" will go down very slowly as it adjusts to your new average.