If you have already been pulling your trailer wit another vehicle, the Following probably will not apply. If you have a new Trailer or just Replaced your brakes then the following May apply. I myself had this problem with a trailer when I had the Axles and Brakes upgraded. No one informed me of the following even the company that did the upgrade after several times trying to figure out what was wrong with my trailer brakes not stopping/or holding the trailer. I found out most of the information below from Dexter and followed their procedure. Now my trailer brakes work a lot better. Next time I will probably go with Disc Brakes on the trailer......
According to a couple of places around my neck of the woods and Dexter tell me after you
initial burnish in you brakes when new, you should be able to manually activate your brakes or pull the emergency break away pin on the cable and your trailer brakes should hold your setup without rolling when in drive at idle without your foot brakes applies. If it does pull your trailer with the break away pin pulled, you have a brake issue on your trailer... Check and compare the trailer controller on your dash when in Drive just sitting there without you foot brakes applied and then compare it to the Break Away Controller. That way you can do an easy brake inspection to see of they can activate. Something in the back of my mind keeps telling me that your Trailer brakes by them selves should hold you setup while in gear without applying your truck brake petal. I do know when ever I was stopped in my big truck I had to do this with my trolley brakes in order to pass Inspection.
If your trailer is empty like a flat bed or utility trailer you could lock up one of more wheels when breaking if your controller is set to high.. The heavier you have it loaded the harder it is.
Here is a link to what
Dexter Recommends.