Finally, i found out that the problems (only the last point and that point wasn't at the right place) had nothing to do with my geojson file : i created a new layer and by default it had projection EPSG:3857, but i needed a projection with GPS coordinates => EPSG:4326 see this . With that change, every point is now displayed. I guess with the wrong projection, all the points where somehow stacked, and then only the last one was displayable.
In Openlayer > Layers
- Use Projection EPSG:4326 (instead of EPSG:3857)
In Openlayer > Map Use
- Map Projection EPSG:3857 (Default)
- Display Projection EPSG:4326(Default)