cannot move on

[riley50]

Hi - I have completed 'trench warfare', accepted the challenge and completed 'grave digger' then ran through 'trench warfare' again.
I am now confronted with the challenges menu showing two levels and a challenge completed. The choices are to replay any of the three sessions or go back to the main menu.
Continue will take me to my last saved game which was in 'trench warfare' just before completion. There is no way I can see to move on to 'the fortress' level. Any help would be appreciated.