Good day,
Before launching the connection:
1- Tick or untick runway follow contour terrain (you must have the same settings here, the change take effect after a restart of X Plane).
2- You must be at the same place (same mesh, same scenery, same apron or runway).
3- Same AIRACS for both pilots. If you want to load a prefilled flight plan, you need it in both coroutes folder.
4- If you are master, you friend must launch X Plane without any weather (X Plane, NOAA disable, Ivao or Vatsim disable, FSGRW, etc..).