Not clear that you have done the usual brake controller "setting-up" drill, to establish just when the trailer stops first, and does not skid. (Although you imply that)
You need some clear road and time to fiddle with the settings, doing it several times as a "trial and error" job.
After, if that does no good, then you can get out the DMM and check for volts and amps and all that rigamarole. (You might be there now, but not clear)
1. 1991 Oakland 28DB Class C
on Ford E350-460-7.5 Gas EFI
Photo in Profile
2. 1991 Bighorn 9.5ft Truck Camper on 2003 Chev 2500HD 6.0 Gas
See Profile for Electronic set-ups for 1. and 2.