Sometimes new trailer brakes will lock up right away. Sometimes they will need to be adjusted or burnished in. A good first step is to have someone put their ear next to the front wheel while you hit your brake controller, to make sure they hear the brakes buzzing on both sides.
Here is a video on burnishing in your brakes:
Here is a pdf document on burnishing in your brakes
Here is a video on adjusting your electric brakes: