Dronekit send waypoint command not executed

I used DroneKit in Python to connect five UAVs to do the simulation on the ground station. Using ArduPilot firmware V4.2.1, they can move together in GUIDED mode when I send them a navigation point command together. However, in some cases, some or all of them will hover mid-flight, and they won’t move no matter if I send the point command again. They can RETURN NORMALLY WHEN CHANGING THEIR MODE to RTL, BUT THE SEND POINT COMMAND will no longer work. The MAVLINK message used by the waypoint command is SET_POSITION_TARGET_GLOBAL_INT