Sharing a file

In a meeting I could not share a file in the chat.  i could not even select one.  The 'My Computer' option was faded and whilst i could click on Dropbox etc nothing happened and no other screens were opened.

My account is set to allow file sharing in meetings (although it says it is locked by admin it is locked as on) and I can share when it is just me in the meeting but when others were on the call I could not do it.