I have 1 GFCI in the bathroom On the 1st floor. The line side is sourced from the main board, and the load side has 3 standard duplexes in the 2 bathrooms on the 2nd floor. After moving in to test the 3 outlets on the 2nd floor, we found that the GFCI always trips when an appliance is plugged in any of the 3 outlets - we can never use any electrical appliance with these 3 outlets! We had a further check and noticed:
The white line from the load side of the GFCI on the 1st floor is capped and not connected. When connected, the GFCI trips.
Without connection of the white wire, when we plugged a voltage meter into each of the outlets on the 2nd floor, the meter was on; but when plugging in any electrical device (even a cell phone charger), the GFCI trips.
The wiring of the 3 downstream outlets connects to each other is normal.
For downstream outlets from a GFCI to work correctly, both the hot and the neutral on the circuit have to pass through the GFCI. A GFCI works (and this is a simplified explanation) by detecting the difference in current passing through the hot and neutral. If it has current moving through the hot and nothing through the return, then it trips because it detects that as being an alternate ground path. I highly suspect that the downstream outlets are either connected to a separate neutral or ground somewhere.