OK - I've just checked and modifiers aren't supported on proxy objects. Therefore even though it'll appear as though it's working for the "preview mesh", It won't render.
I would recommend that either you load the geometry into the scene or use XRefs, or you could apply the modifiers before converting them to proxies. Forest Pack doesn't actually require geometry to be a proxy though, it's pretty must just as efficient scattering scene geometry.
Cheers,
Paul