Incomplete Mission

[WhiteKnight77]

OK, I started a mission in a career and was sent to patrol off Hokkido and went to the area on the map were I was supposed to patrol. I sailed around the area a couple of times before heading through the strait between Hokkido and Honshu to get to the west side of the island. I patrolled both to the south and north, sinking ships here and there. Sailing north to go around the north of Hokkido I ran into a small convoy and sunk everything, including 3 destroyers though I took some damage from some depth charges (ouch). After finishing all the ships off, I continued around the north of Hokkido and returned to the original area.

Even after that, I still got the mission incomplete report in the Captain's Log so I decided to head back to Pearl and see what happens when I dock. I did save my progress several times though I haven't tried to restart it yet, but when I went to quit and return to the main menu, I got mission fail. On a good note, I sunk over 51,000 tons on one trip with no refits.

What am I missing to complete the mission? All help would be appreciated.