Are you connected to a gateway?
The “join: ok accepted” message unfortunately does -not- tell you, if you have connected to a gateway.
for testing purposes you can also use iot.semtech.com port 1680/1680 and then see if it shows up here:
http://iot.semtech.com/gateways/
I’m also experimenting with www.loriot.io .For now it looks to be the only one who supports confirmed messages.
Problem with them is that every transmission includes a frame counter, and after modifying the sketch or reconnecting power, the frame counter is set to zero, and messages do not show up anymore.