Depends where you are. You don't get missions if you hang around inside area maps like Drozana, DS9, Around drozana in the are space and so on. You only get them in Sector Space, ESD, Sol and SFA. Also there's a timer on the doff screen telling you when the sector/shipboard assingments will refresh.
Assignments in sector space refresh every 4 hours currently, shipboard assignments refresh every 60 minutes - you do not need to change maps to see the refresh, it just happens.
There are also some system / ground maps that have assignments (SFA, ESD, Wolf 359, Romulus, Earth orbit etc) which also refresh every 4 hours. Though curiously there are never any assignments in Qo'nos orbit.
If you are not seeing any, it is likely you are on a map that doesn't get assignments, try going to one of the above.