So I've noticed that once I can't equip any of sicarios who have nothing nor can I remove any sicarios from a specific team. I am able to get the Green + button back for the sicarios by first locating which sicario on which team I want to equip, then clicking on the "Sicarios" tab above then clicking the "Team Management" tab again. The + button appears and you can add the weapons or vehicle you want but it doesn't stick.

When I had this, there was a Sicario with the same name already on said team or under the 'Sicario' tab.

Maybe it's the same for you? Otherwise I wouldn't know and i'll just let Diablo handle it.