I'm thinking you may not have them setup correctly. Mine are set to around 5-6... They were adjusted on a back country road to start slidding the trailer brakes and then backed off a number from that sitting.
The only time I have a problem with mine is when they have sitting for awhile and I first use them... They stop everything OK but is is usally a very rough stop.
Out of habit now I will hook up the trailer and when I leave my yard I have a STOP SIGN about three hundred away and I will use the manual slide lever to stop my truck and trailer at the residential stop sign. This usually cleans up the sitting debress and possible moisture that has accumulated on the pads... Then I turn on my lights and flasher unit ON and do my safety walk-around the trailer checking all the lights...
Also tells me my brakes are working good before getting on the big roads...
Roy Ken