I have wondered the same thing as
Stuck_in_an_ATR for the past 4 years.
"no optimal" happens if the FMGS calculates that you cannot save any fuel by climbing to the new opt FL. i.e. too close to TOD.
But it still shows "no optimal" with 4-6 hours to go. And if you force a step climb further down the road, it will show an increase of EFOB at destination.