My main settlement Sancutuary Hills only has 8 settlers, and seems to be growing slowly, even though I have built lots of beds and have a high defend score of 33. Food is 18, Water 15, and there are 24 beds. Power Level is 13 and there is a recruitment signal going on for a while.

Yet it does not seem to grow that much. When entering the Data tab on my Workshops, I see an exclamation mark on Sanctuary Hills, yet it is not clear to me what needs fixing.

For the other settlements, there is also an exclamation mark at the low ressource, most often low defense.

How to grow my settlement fast?