TPL5010: RSTn signal disappear

Hi:

           I use TPL5010 as a watch dog to reset my MCU.  I connect WAKE ,DONE ,RSTn  to my MCU .and set reset time is 8 s.

      But my mcu don not sent DONE signal to TPL5010. I find out:   I can capture  several WAKE signals at  first , I also can capture

      RSTn  signal every 8 s ,but there is  only WAKE Signal ,no  RSTn  signal ,I don not know why ? Can you help me why  RSTn  signal

     disappear ? Thanks you !