@Loïc-Gangloff
Hello Loic,
The most likely cause is that the total activity time you recorded over the 8 days is greater than the available activity storage space in the watch. So the first three days will have been overwritten with the most recent days.
If you go into the watch logbook and open one of the activities that won’t sync, after it has loaded do the graphs show for pace, altitude, heart rate etc? If they don’t and you only have the numerical summary, then the underlying data is no longer there and so can’t be synced. If the graphs do show, then it should sync, in which case I don’t know what the problem is.
Based on my experience the watch can store somewhere between 70-100hrs of data, depending on exactly what is recorded (e.g. less if you record HR). I once lost the first 60hrs of a 100hr race when I split the recording and forgot to sync the watch before starting the remaining 40hrs.