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TCP receive window size doesnt change

hi..
im about to make experiment in TCP receive window size as chours project.
i im trying to check receive window size behaivior as function of number of current download
unluckily... the calculated window size didnt change at all...
after i lost my sane i connected my pc to my smartphone by hotspot and then window size changed in every pocket
my questions is:
1) what was the diffrent between the two download ways that TCP rec window size behaived
2) how can i case window size to change.. what is mainly influenced by.. if its about the TCP receive buffer how can i impact it to see changes

thanks