[rtmpdump] why would rtmpsuck save small files while output full length data back to the browser

hello, I was just trying to save recorded meeting from adobeconnect using the tool from http://www.nirsoft.net/utils/rtmp_dump_helper.html along with rtmpsuck windows build from http://rtmpdump.mplayerhq.hu/download/rtmpdump-2.4-git-010913-windows.zip The result was a bunch of screenshare_xx_xx files all 13 bytes in total starting from "FLV |" (apparently flv header) but the actual data was missing. At the same time the browser was playing them back just fine. Am I missing anything obvious about how rtmpsuck should work?