Hi Márcio!
The Python agent should log any issues it has sending data to an elasticapm.errors logger. Depending on how exactly your logging is set up (which tends to be different from Flask app to Flask app), you need to add a handler for this logger to see its output.
BTW, which app server do you use? If you're on uWSGI, make sure to enable threading.
Failing that, your requirements.txt and initialization code could indeed help with resolving your problem 
Cheers
Beni