hmm ive only just started playing the game but my guess would be either the number of communication towers or the number of shrines you own determines the number of engagements you can have per day if you have leaned too heavily on foundries and such i can guess it might cause this problem.
This is however just a wild stab in the dark i thaught id share with you since you havent gotten any other reply i hope im right
Also it may be to do with skipping dialogue during missions see the topic angel forge woes above i dont know if your in the same location but its possible the same solution applies
[Edited by Ramezrh, 4/10/2009 5:53:16 AM]
Thanks for the info, but actually I'm only left with one mission 'Last Stand' but the defend the tower or foundary missions are annoingly never end.