Have you noticed if always the same machines failed to pick up the job or it is on a random basis ? Same machine
What about rendering a simple scene with a few primitives without any textures proxies and etc? Just increase the render-times by increasing Image Sampler subdivisions to get all machines connected to the job. I will try this
How many machines participate in the rendering ? What about if you use only half of them ? 7 machines at the moment - I will try using fewer
Make sure that all machines have the same Vray/3DSMax and etc version installed. They are the same
What OS do you have on your server-machine - the one where the projects and assets are stored ? Server machine where files are stored is W2003 Server
Are the spawner’s started as a service or manually ?