I have recently been working on a mediation analysis, in which inclusion of an additional variable (say, C1) implies reduction of the effect of a focal predictor (X).

Now, moving further, I add a new variable into the model (say, C2). It's impact on Y is negative; however the impact of the focal regressor (X) on Y is increasing --

**what is this indicative of?**

Assuming inclusion of C2 decreased the impact of X1, I would suspect it acts as a suppressor (considering C2 has negative impact on Y). But I am a bit confused by C2 boosting X's impact. Note, there are no signs of multicollinearity. Predictors are correlated at .3 max (both, VIF and condition number tests are at minimums -- 1.5 and 3, respectively).

Your comments are greatly appreciated