Custom MAVLink Message doesn't send

Hello everybody,
I want to send a custom MAVLink message with my Pixhawk 1.
I use 1.8.0 px4 and the px4fmu-v3_default to build the firmware.
I followed the instructions for MAVLink messaging and so far, I can publish my custom uORB topic and start a MAVLink stream on my TELEM2 port, sending my custom MAVLink message.
The firmware builds fine and I added a console log in the mavlink_messages.cpp in my custom mavlink stream class, right after sending the message with “mavlink_msg_…_send_struct(…)”
The console shows the print statement, so I think the message is sent properly, yet when I connect the TELEM2 port with my PC and use a terminal program to show the received data in hex, I recognize that only the heartbeat messages is received.
The mavlink stream is started in the rc.txt:

mavlink start -d /dev/ttyS2 -m custom -b 115200
mavlink stream -d /dev/ttyS2 -r 50 -s CUSTOM_MSG

I didn’t changed the custom mode, so it only streams the heartbeat message, therefore i added the second line.
So far I think the problem is somewhere in the custom header file, probably when calling _mav_finalize_message_chan_send(…)

I would really appreciate your thoughts on this!