jim, et al,
Actually, that's not true. I have a 7 pin round connector on the back of my Dodge truck and I have a 7 pin round to 5 pin flat connector adapter, although mine is not exactly as CassBass described.
The center round pin is the back-up lights wire.
The bottom right hand side flat pin and the top right hand side flat pin are the only ones used specifically for electric brakes.
The adapter I have from the 7 pin round to 5 pin flat only has 4 flat pins installed out of the 6 spaces on the round side. They are for:
1) right hand turn/stop,
2) left hand turn/stop,
3) running lights, and
4) ground.
The other two spaces that don't have pins installed are for +12V and the electric brake.
More than likely, I'd say that you either don't have a wire hooked up on your trailer side to the solenoid, or the wire on your truck side isn't connected properly to the center pin. Should be pretty easy to check with a multimeter. With the parking brake on and the rear tire chocked and someone else having their foot firmly on the brake pedal, put the positive pin of your multimeter on the middle round pin, and the negative pin of your multimeter on the lower left hand flat pin (-). Have someone put the truck in reverse. You should have at least 12VDC. If not, check the single wire going into your solenoid on the trailer side - probably a single blue wire.
All the best,
Glenn