If the welder doesn't have a polarity switch try switching the leads where they attach to welder.
Mike A

7018 works best with DC reverse polarity. For steel of unknown alloy, 7018 is a good choice. Work must be clean and dry, electrodes must be kept (stored) absolutely dry, very low humidity.

Sometimes when my rods wont start I stick them to the metal and let them smoke... Pre heating the rod makes them work so much better...

I have found some rods have the coating covering the very end of the rod. This would insulate the rod from making contact and arcing. I asume you know about setting DC polarity and are holding a short arc. The rod usually produces an excelent bead with min.effert.