Sometimes this problem can be corrected by adding a few drops of vinegar and stirring very thoroughly. Smooth out the surface of the paste in the bowl and add just enough water to completely cover the paste. This thin layer of water keeps the paste from drying out by preventing contact with the air. Remove the mixing bowl from the cooling bath. Give the paste a stir and gradually add small amounts of warm water until the paste stirs easily and becomes smooth, elastic and somewhat shiny.
Proper consistency is very much a matter of personal preference based on experience.
However, good nori paste should make a kind of plopping sound when it is scooped up and then dropped back into the bowl. Also, if you draw up a thin thread of the paste, it should break off easily, i.e., it should be like taffy. Lastly, if you form it into a mound shape, it should be relaxed enough to revert to level within 10 to 15 seconds. The paste is ready to use. These cones work on the same principle as a cake decorator.