Assuming the brakes are in good working order, either the trailer is too heavy for it's brakes (near or over the axle weight ratings), or the controller and all the wiring thereafter isn't providing enough juice to the trailer's 6-brake system. Could be a wiring issue: wire gauge too small for length of run, or bad connections.
Weekend Warrior, and others, made alot of giant toy haulers, which were built on chassis that were too small for their weight... They were often already overloaded as they came out of the factory, before anything was loaded into the cargo bay...
In that case, you may need to upgrade to heavier rated axles (and their correspondingly larger brakes).