The behavior is that it would not start sending data at all, for temp and/or humidity. Debugging directly with an Arduino sketch, we saw it was hanging on the wire.endTransmission(), in the readData function of the driver.
Searching on the web, we came across a few forums where folks suggested to add a delay into the "wake-up" block of the readData of the driver. Other were suggesting a delay between each measure in the loop function. None seemed to have an effect for us.
It auto-magically started working and the rostopic is now returning the data properly. Until the next time... Any idea?