NMR calculations with NWChem quantum chemistry program
Those calculations are tricky because some functionals work well, others don't, some basis sets work well while even a better basis might give bad results and so on.

We did some studies in the past using the GIAO Method in G09 with B3LYP and 6-311+G(2d,p) als basis for the NMR calculations and M06-2X/6-311++G(d,p)for geometry optimization. And that worked quite well. However, we always used solvated systems (PCM) which seems to be the common way to do it. Try to reoptimize in solvent and run the NMR calculation in solvent if that's possible in NWChem.