I am using the PROCESS macro to do a basic mediation analysis, with variables X,M, and Y. All are continuous. I know from theory that X and M, and M and Y respectively, should have a relationship. I am certain from the theory that X is causing M, not the other way around. Indeed, in my data when I do a regular linear regression, paths a, b, and c, are all significant. Standardized Betas for the 3 paths are 0.76, -0.17, and 0.16 respectively.

This leads me to expect that when I do a mediation analysis, at least one of paths c' or b should have a significant relationship. However, this is not the case. In the PROCESS results, both the direct and the indirect result are not significant. My suspicion is that X and M are so strongly related (unstandardized B=0.76), that both path b and path c' are suppressed by this. Path ab is quite a bit larger than path c', which does point at a strong degree of mediation. However, since path ab is not significant, I don't feel that this result is of any value.

Is my interpretation correct, and is there any way I can deal with this?