How many different types of nectarine were you attempting to graft? It may just be that you had trouble with a particular variety, which could come down to the scionwood. I grafted a bunch of peaches and nectarines this spring and the results were anything from 5 out of 5 to 0 out of 7 (perfect for Early Crawford and zero from Zin Dai Jiu Bao, with many varieties in between). I suspect it comes down to the wood from some being older, less vigorous, or just not as good in some unknown way. I didn’t do too many nectarines, but off the top of my head I remember that at least one of them was near the top (good) end of the spectrum.