I've had this happen a few times, and most of the time I noticed there were multiple Ops buttons on the edge of the screen. Once each one of these has been clicked through, usually I was able to complete the mission. However, the issue still stands that this is not very obvious and this doesn't fix the issue every single time.
type /lfg and you'll never be made to join a team again... it has solves 90% of all problems I've had with missions.
the other 10% was solved by never clicking "not now" when a BO asks me if I want to warp out.