Global Tag not showing any tag from subsites

Dear all,
tags are not being displayed on our multisite main site (http://eui.wpengine.com/tags/)
while there are posts with tags in the subsites (http://eui.wpengine.com/emanuele-strano/2015/09/09/testtete-estet-etw-tewte-ew/)

The tags are: test, test2, test3 and test4

Thanks for your kind help