Networker always do full backing up even if with incr mode schedule

I'm doing NDMP backing up with incr mode schedule, but I found that the job always do full backing up. I have checked the Networker log and i found that the NDMP log part showed:

Date of the last level '0'. Thu Jan 1 08:00:00 1970

Date of the this level '0'. Tue Jun 25 10:48:00 2017

Even if I run the job many times, the log shows the last level always being "Thu Jan 1 08:00:00 1970". can anyone give me some suggestion?

消息编辑者为：HePeng