Possibility 1: You need a wire from the booster ground to pin 11.
Possibility 2: You need one detection section wired up and working.
Leave it connected, don't remove the wire and then connect up just
Transponding. The board needs the DCC track signal for Transponding
to work. My guess is that you probably have got detection working and
then just added the wires for Transponding, but I mention this just
Possibility 3: Each time you try something, reverse the a and b wires
at the booster. If it is not working, reverse the A and B wires right
at the command station/booster and try a second time. (I had a RX4
with the printed side backward from the others, maybe you got one
like that too.)
The direction the wire passes through the RX is important, as well as
which track feeder wire is connected to "A" on the booster. Each time
you try it, also swap the loco from end to end on the track. If a
loco transponds in one orientation but not the other, then you are
close to having it work, but still have a polarity issue as far as
the direction the wire travels through the RX1 or the polarity of the
booster output. When everything is connected right, a Transponding
car or loco will work regardless of orientation on the rails.
Just to confirm, the dt400 find key does indeed seem to be a good
check to see if Transponding is working. I have used it often.
One last thing if all the above still does not help. Plug your
throttle direct into the command station, connect the BDL directly to
the other loco net socket on the command station. (eliminate all
other devices from the LocoNet). Make sure the loco net cable is a
good cable, A Digitrax factory cable is a best choice. Check the
cable against the diagram in the big system manual and make sure the
colors and polarity match the diagram exactly! Mis-wired LocoNet
cables may work fine for most other devices, but will not work at all
with Transponding. Don't ask me why, I don't know why Transponding is
so picky about the LocoNet cable, must be another polarity issue.
If all of the above still does not help, then let me know. I'll hook
up my spare command station and a BDL and experiment. Perhaps some
good clear photos of your test setup could be posted or emailed. I
live in Florida. Planning a vacation? Maybe you could stop by and
bring your stuff.