If incoming messages forward to the MightyText Web App, but any messages you send from the web app don't work, try the following:
(1) Make sure your phone has the latest version of the MightyText Android App (from Google Play)
(2) On your Android Phone, go to the MightyText app
(3) On the bottom of the screen, tap the "Unlink phone" button
(4) Select your Google Account that you want to use (same one that you use on the Web App)
(5) Double check that your phone has both a data connection, and a voice signal.
(6) Try sending a message from the MightyText Web App, and check your phone to see if it sent