First, check that you have the latest version of the firmware. Some older versions do not calibrate automatically the ground sensor values.
You can try calibrating the sensor values by hand:
- To calibrate the black level, with Thymio's ground sensors positioned over the track, press both forward and backward arrow buttons simultaneously. - To calibrate the white level, with Thymio's ground sensors positioned over the white area, press both left and right arrow buttons simultaneously.
If you draw the track yourself, make sure it is sufficiently wide (4cm) and with high contrast (black on white is best). If some turns are too sharp, the robot can lose the track.