Decision tree too small
Have you changed the max depth parameter? This parameter limits the depth of each tree. If you have set it up to max_depth=1 you wont be able to grow your tree further.

From the documentation: 


  max_depthint, default=None
  The maximum depth of the tree. If None, then nodes are expanded until all leaves are pure or until all leaves contain less than min_samples_split samples.


Also, it could be that you have your target on the train and your tree does not need to develop further.

Or that you are training with a small number of rows or columns