C++11 += operator not working when RHS has more than one string or character

I am using c++11. When I use the += operator for concating strings or characters, it does not work, however =, works.

e.g. I used all the below test cases independently, i.e. separately.

    string s="abdddddd";
    string ss="";
    ss+=s[0];//working
    ss+=s[0]+s[1]; //not working            output: Ã
    ss+="hi"+s[2];  //not working           no output
    ss+='d'+'c';  //not working             output: З
    ss+="hi"+"string"; //not working        error: invalid operands of types ‘const char [3]’ and ‘const char [7]’ to binary ‘operator+’

    string another="this";
    ss+=another+'b'; //working
    ss+="hi"+another;//working
    ss+=("hi"+s[3]); //not working
    ss=ss+"hi"+s[3]; //working
    ss=ss+"hi"+"this"; //working

Also adding brackets doesn't work. So, I want to know why it doesn't work with strings, it works with adding integers.