The problems you are experiencing may be due to how the viscous couplings have been stored.
They should only be stored in an upright position,it's acceptable to lay the coupling flat for a short period during installation but never with front face down.
Don't ask me why this is,probably to do with the fluid distrbution in the coupling and the lockup situation you experienced may have resolved with more extended use,that is if the fan blades didn't part company first.
Geo.