So i have this problem with the main quest "With our powers combined". When the Prywden gets shot down, enemy vertibirds begin to spawn to attack the castle and the quest "Defend the castle" starts. Each vertibird and all the enemies within I destroy counts as a 10% completion of the quest.

The problem is that every [censored] time I get to 70% the enemies cease to spawn. I have tried everything from fast traveling, waiting in game for 30 days, about 30 times of reloading my save file right before I start the mission, searching for nearby enemies if they perhaps got separated...basically everything and nothing works.

Please do not post the link to fallout wikipedia for this quest because all suggestion about solving this issue on that site are pretty much useless. Does anyone know how to get around this?