The Synergies can only be activated by one person in most cases (not sure about the Veil of Blades? one that allows other players to go into Stealth if more than one can activate that one).
So, what I ask is, are you sure the Synergy is not being activated at all by someone, or is it that no one can activate it? Also, if you make sure only one tries to activate the synergy (for testing) does it activate easier than if two are trying?