RitaShen wrote: some of reply I see in the system is blank, some of reply is complete.
How does a "blank" reply looks like? You have incomplete reply with just an id and no answer saved? If you have a visitor which isn't answering any question, you have such a "blank" reply.
I never had a survey without such "blank" reply. So it can be quite normal to have "blank" replies.
If you still feel there is an issue, you should contact the support.