Make sure both of your accounts privacy settings and linked accounts are set to public. Keeping your XBL/PSN private will prevent the mission from popping up. As soon as my friend changed theirs, the mission was in the tower instantly. Took us 20+ hours to figure it out but there it is. Good luck guys, hope this helps solve your problems!!!
EDIT: Clarified a few things.
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This also helped a few people that I play with.
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Really?