title: Rolling window produces no effect on dataframe
body: <p>So I have to perform a rolling window to a set of rows inside a dataframe. The problem is that when I do <code>full_df = full_df.rolling(window=5).mean()</code> the output of <code>full_df.head(2000)</code> shows all NaN values. Does anyone know why this happens? I have to perform a time series exercise with this.</p>
  <p>This is the dataset: <a href="https://github.com/plotly/datasets/blob/master/all_stocks_5yr.csv" rel="nofollow noreferrer">https://github.com/plotly/datasets/blob/master/all_stocks_5yr.csv</a></p>
  <p>This is what I have:</p>
  <pre><code>df = pd.read_csv('all_stocks_5yr.csv', usecols=[&quot;date&quot;, &quot;close&quot;, 
  &quot;Name&quot;])
  
  gp = df.groupby(&quot;Name&quot;)
  my_dict = {key: group['close'].to_numpy() for key, group in gp}
  
  full_df = pd.DataFrame.from_dict(my_dict, orient='index')
  for i in full_df:
      full_df = full_df.rolling(window=5).mean()
  </code></pre>
  <p>An image of the output:</p>
  <p><a href="https://i.stack.imgur.com/b75dC.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/b75dC.png" alt="enter image description here" /></a></p>