ImportError: No module named flask.ext.sqlalchemy
You should be building your python apps in a virtualenv rather than using the system's installation of python.  Try creating a virtualenv for your app and installing all of the extensions in there.
I am deploying my flask app to EC2, however i get the error in my error.log file once i visit the link of my app.
My extensions are present in the site-packages of my flask environment and not the "usr" folder of the server, however it tries to search usr folder to find the hook

File "/usr/local/lib/python2.7/dist-packages/flask/exthook.py", line 87, in load_module

It is located in 

/var/www/sample/flask/lib/python2.7/site-packages

How to get over this issue?