2015 Ram owners may be experiencing the same issue as the OP. Ram went to a new ITBM that uses ABS and and accelerometer to adjust the pulse width modulation to the trailer brakes.
2015 Ram owners with 8k and up trailers are finding they cannot lock up their brakes using the slide bar. Trips to Ram result in being told everything is functioning as intended and it's a trailer issue. The resulting trip to the trailer dealer finds the brakes are properly setup and functioning properly.
My father's 2012 Ram with ITBM locks my brakes no problem. I can't lock my brakes at 10 gain on light or heavy electric.
There is 20+ pages on this and the associated frustration with RAM and the dealerships over on a Ram forum.