why timeoverlay doesn't work

[方明]

hi all
        
   I gst-launch a pipeline "...h264parse  !  avdec_h264 ! videoconvert ! clockoverlay  !dshowvideosink "


but  I cannot see a correct displaying of clock on the video .I just  see some little span frames on left top. 


textoverlay  is the same so.


When alternatively I use "textoverlay text=xxxxxx", I can see 6 little span frames. 


Any  help ?
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