is box D on form 6781 checked? that's an election to carryback some or all of these losses to the 3rd prior year. the amount to be carried back is on line 6
any other boxes check?
there may be another reason - if you had other capital gains that offset the losses on the 6781, then there are no losses to carry forward. look at schedule d for 2017 line 16 if it's positive no loss carry forward.
other than these, ask your accountant to explain. you paid him i assume so he owes you an explanation
