No potential across output of H-bridge
It sounds either (i) like your motor driver is broken or (ii) your code does not operate the H bridge properly.  Check which digital outputs pins of the Arduino control the H bridge and make sure that they do not short out the H bridge.

If you want to try another H bridge, this one is suitable for your application: https://www.pololu.com/product/2990 .  I have worked with this before and it's very easy to use.