My 15,000 lb 5th wheel with only two axles will not lock up the brakes. I was told this is normal due to the weight and how the magnets grab to activate the shoes.
I was never happy with the brake operation, I always felt they were too week. The meter showed the correct voltage and amperage at the connector and I always kept the brakes properly adjusted.
I bought a new truck a few years ago with an aftermarket brake controller installed and was surprised at how much better the trailer brakes worked. They still won't lock up but they have a lot more holding power from this electronic controller. It's a DrawTite brake controller.
I've been very happy with the brake performance since switching to this new truck.