External hard drives not consistently recognized when USB Hub plugged into Plugable Dock

When installing USB 3.0 Hub onto Plugable 3.0 dock, not all my plugged in drives are recognized at startup. On the occasion that all drives (6 of them) are recognized at first, they drop out or system hangs when transferring files. Not the hub as I don’t have the problem when I plug the hub directly to the Dell notebook.