It seems that Windows prevents from such a focus stealing by default so as to defeat annoying adware ...
A solution can be to set the windows registry key
HKEY_CURRENT_USER\Control Panel\Desktop\ForegroundLockTimeout
to a decimal value of 0
then calling
hComServer.Activate
now activates and brings the window to the front but sometimes several calls seem to be necessary
(NB : reboot required after registry modification)