The first thing to make a perfect whipped cream is to chill everything. Keeping the temperature low helps incorporate air into the cream for a longer time, resulting in stabilized foam. So, not maintaining a cold temperature is the foremost reason for cream not getting whipped. If you are whipping cream by hand, I strongly recommend chilling your bowl, whisker and cream before you plan to make whipped cream by hand. Place your bowl and whisk for at least 20-30 minutes in the freezer BEFORE using them to give them enough time to chill. Also, do not forget to use chilled cream, not frozen.