Witch Doctor Artifact upgrade

Stanna_Unruh

There are several of my artifact "Helpers" that won't upgrade.  I have three stars on all the levels and it WONT UNLOCK the higher level of helper?  Is there another way to Upgrade a character like the Witch Doctor?  Am I missing something?

Baz_James

What does the score beneath the Witch Doctor say? To get the upgrade you need 135/135. If it's any less than that you've missed a star somewhere or you haven't reached all the necessary levels even if you think otherwise..