My answer is based on this other answer: Trouble with TensorFlow in Jupyter Notebook. I had the exact same problem, and I solved it by doing this:
Once your environment is activated, run "which jupyter" and verify if the path returned points to the /bin folder under your environment, something like /my_environments_path/my_tensorflow_environment_name/bin/jupyter. If this is already the case, your scenario is different from the one I had;
Run "which pip" and/or "which pip3". Odds are, you are executing pip or pip3 from a different location, so Jupyter is not being installed inside your environment. In my case, Python version was 3.6, but pip3 was being called from a different location. I ran "pip install jupyter", since pip was inside the /bin folder in my environment. Once I called the correct pip, I checked again the "which jupyter" command, and this time Jupyter was under my environment and I could import Tensorflow inside my notebooks. If no pip's path points to your environment, install pip inside it by running "conda install pip".
I hope this works, along with the additional information on the link above.