I have a research design with 3 predictors and 2 mediators, testing on 2 different samples with multiple regression analysis. For my first sample (N=70), variables(mediators) entered at the second step predicted significantly the outcome, but only one of them had a significant coefficient. The IV's had non significant coefficients at step 2, and the beta's decreased in size. In my second sample (N=100), same set of variables, at step 2 the proposed mediators didn't predict significantly the outcome and none had significant coefficients. The IV's also had nonsignificant coefficients at step 2 and they decreased in size. Is mediation supported for the first sample? What about my second sample, what can I say about the results if none of the mediators was significant, can I draw a valid conclusion and still use a mediational design?

All the variables are positivelly correlated and there may be some multicollinearity problems for one of the mediators in each study. Some of the predictors have high correlations with the mediators, above .6.