Hi pela03 I had this problem as well but after many trials I think I may have found the solution. You just need 4 steps:
If you are fortunate enough to have any device such as iPhone 6, that is compatible to the Freedrum Sensorware app, then congratulations, but if not, try and ask a friend to download the app for you with their device so that you can proceed.
Open the app (with bluetooth on), turn on all sensors (make sure they are fully charged), and connect them to the phone.
If you shake the sensor, a music note will appear in its respective sensed device, then set 2 of them to be foot pedals (make sure you remember which 2 you have set as foot pedals)
Now disconnect everything, and try the same thing again with mac with the given procedures by Freedrum, this time, strap the sensors to your feet first, then press the square button for both feet sensors WHILE YOUR FEET ARE ON THE FLOOR (I’m not sure if it’s better if you do this AFTER you have calibrated your sensors on your drumsticks with the given procedure first, but I do it every time just in case)
This method solved my problem and it doesn’t happen again, hopefully it helps you as well, have fun drumming