matt_m

In that case, you need to center the decoupler along the length of the stack betterâ€â€optimally, directly through the center of mass.

Its a bug in KSP. When there are parts strutted together "across" a decoupler, the decoupler's ejection force is not applied to the decoupled part. While not always as effective as using seperatrons, you can install Kerbal Joint Reinforcement (v1.2). If I remember correctly, ferram4 was able to work around the zero decoupler ejection force by disconnecting all the struts between the coupled parts, then firing the decoupler.