Please help!!

[Rockpig]

Here is the setup - Brand new Toyota Tundra 5.7 with two package (10,300) and 20 ft. trailer, both axles have brakes, breakaway, wood deck, 2" ball, hammerhead hitch (7k),Tekonsha P3 brake controller (new). Trailer is used (new to me) and was told everything worked good. I have no idea of wo manufature the trailer or axles. I figure the trailer weighs max 1700 lbs.

Today was the first towing after installing the brake controller. I tore down the brakes, cleaned them up and everything seemed to look good. Hooked it all up and tried adjusting the controller.
As per the instructions for the P3 controller it says to adjust the brake shoes out until the pressure of the lining against the drum makes it VERY DIFFICULT to turn. So this is what I did.
As per the instructions I started the controller at 6 and pulled forward and pulled the manual lever on the controller. The brakes did not lock up. Drove home (22 miles) and kept adjusting up while driving. They brakes had plenty of time to warm up. I finally got one tire to lock up out of the 3. The controller was at 10.8. Pushed it up to the max of 12.8 and could only lock up that same tire. Tried adjusting the 3 remaining non locking brakes to the point I could almost not move the tire, no difference. (Tire pressure same in all 4 tires).
My main concern is that the trailer us unloaded, my 4Runner may weigh about 5k (need to weigh it). If I can only lock up 1 tire at 10.8 and no ohers at max (12.8) what is going to happen when I load my truck on there? I can only imagine the brakes will be worse.
I have maybe 10 hours over the next week to get it all working properly. I still need to load the truck and try towing the whole thing so I can get used to it's road manners.
What else should I do to get all 4 to lock up?
What else should I check?

[MTeator]

Check the ground connection.

Also, I would highly recommend taking the trailer to a place specializing in trailer maintenance. Unloaded, I set my P3 to 2.5 and loaded with my 2800lb car + extras it's set at 7.5.

My trailer only weighs 1000lbs, but still, yours should be not too far off for the brake settings unloaded.

[TundraBay]

Well either it's mechanical or electrical. See if your 12.8 volts or anything close is being read at the axle at each of the brake magnets. If so, you likely need to either free up the movement or replace one or more of them. But probably it's wiring, a bad connection, or (as suggested, above) inadequate ground.