Those settlers are ungrateful people.
I think if you lived in a post-apocalyptic world you'd be pretty depressed too.
I'm not sure it is possible to get happiness to 100%.
Yeah I gave up trying to please them.. Wow impresive numbers there. 16 shops! They like electricty too so hook them up with TV's radios and lights in their bunk houses along with decorating their walls. Also you will need to help defend the settlement, if you blow them off (like I seem to do on a frequent basis) they lose some happiness.
Oh man, I didn't even think about the electricity. I need to do something about them living by those kerosene lamps!
Well quick turn those jukeboxes on and roll out more rugs!! Sounds like you will get there mate..
You're going to need to do a little exploring, and look for the Picket Fences magazines. Each one allows you to craft new items, and this seems to be a big factor in happiness.
I just found the one which allows me to create house plants, and when I threw a bunch around the settlement homes, happiness went from 68 to 81 (and still rising).
Anything that changes a settlement's appearance from drab to good (paintings, jukeboxes, etc) will also increase the rating.
You'll know when you're on the right track as most of your settlers will comment on how well the settlement is looking.
It's not even possible to get 100% happiness in the real pre-apocalyptic world ...