Hi Coldcall
The Campaigns and missions are all date related.
Post #64 Open Horizons II > Campaign Dates > Bases > U-Boats
To be able to select the next mission after the Baltic Operation you need to end the patrol after it appears on your navigation map, using gmt stopwatch time IIRC.
If you end the patrol before the next mission start date then you will not see it to request it.
Always make a game save in the bunker before starting a new mission.
Peter