Find an empty hard surface parking lot and do 10 slow tight figure 8s one direction, then do another slow tight figure 8s the other direction. Expect to hear a lot of strange noises, its a sign the process is working.

I suspect the friction plates are dry between them. The figure 8s force the plates to separate so the fluid can get between the clutch discs.

If that doesn't do the job, its probably time for a rebuild.