The typical causes of bad braking are these:
- improper adjustment of the shoes (traditional elec brakes)
- poor electrical connections
- altered coefficient of friction (grease/oil where it doesn't belong; on the brake surface)
- badly over/under inflated tires (not a little, but a lot)
- electric over hydraulic are similar except for the nuances unique to the pressure circuit
My RV does not have auto adjusters, and I'm OK with that. I've heard they are not that reliable, and perhaps that's what you're into? I'm sure there's a way to manually adjust them- try that.
It often takes a while to get them adjusted equally. Part of that is because we want to see them adjusted as a visual reference, but we cannot "see" a weak circuit or magnet.
Because we live on a long gravel drive, I have someone drive the RV about 30mph and "lock" the trailer brakes with the controller. Then I watch to see if they all four lock-up at the same time. If not, there's typically one which is lagging; that's the one you want to tighten up a bit. My method isn't fancy, but after a few tries, it works well and is reliable.