If you can't lock them up it could mean a several things:
a. Trailer brakes aren't adjusted properly. Gap between shoes and drum is too large. Here is a good video of how to do it:
b. Poor electrical connections limiting current flow to brakes. This could be oxidized pins on connector, too thin wiring, poor ground, or poor wiring splicing at junction box and/or at wheels.
c. Glazed pads from overheating or not properly bedding in new pads on camper. Glaze can be knocked off with sandpaper.
If brakes are new, give them time to bed in and follow common bed in procedures on the internet. After bed in, adjust them per video above.