I am almost sure it is the anchor text. (Could be other signals as well) This month the link building has generated almost zero results for exact match anchor text. While something already ranks well for "alpha beta" we were unable to influence the rankings for "beta alpha" on the same url.
This came as a bolt from the blue. we had seen a decline in influence from cheaper links since january but it gets tough now. This month was a killer blow. No benefits at all.