First of all, make sure that Headshake is shown in plugins admin. If it’s not, make sure you installed it correctly as shown in this FAQ: https://www.simcoders.com/faqs/headshake-is-not-shown-in-the-plugins-list/
Then, make sure that none of your devices (joysticks, yokes, etc.) repeatedly calls an X-Plane view command (“Glance Left”, “Glance Right”, etc). If this happens, Headshake temporarily stops its control over the X-Plane camera to let the view change properly. If a device repeatedly sends a view command then Headshake stays on hold forever. It does then not show any effect.
← Headshake is not adding any camera effect to my X-Plane
Headshake is not adding any camera effect to my X-Plane