My happiness level is at 10% and I've tried everything like upgrading rooms, putting the dwellers in the location that proteins to them, I have a radio room, I have also tried to put them in the living quarters and dance but they do and still are at 10%. I got pets, upgraded the rooms, and I tried to rush them because I heard that a successful rush will increase it by 10% but still NOTHING HAPPENS. Another thing is when I leave the game it goes up to 15% but when I join it goes down back to 10% and I have no clue what to do. Can anyone help me??
Aside from the post that Ben linked, dwellers often talk about the things they want, for example based on their S.P.E.C.I.A.L. they comment how happy they are with their current job(assigned room) and even how happy they feel if they're in the same room with one of their family members. So, listening to them could give you a little bit of idea about what to do.