Check to see if necromancers have unlisted 'persistence of mind' effect. Archmages have it, and I believe makes them immune to shackling as well.

I bet they do, and I bet that is what is causing them to be untargetable.

All the checking up has too wait until I get home from work, but out of curiosity you didn’t have any issues shackling necromancers?

We've been talking about it a bit in the patch fixes thread, and it definitely looks like you can't shackle Necromancers (all other undead seem doable tho').