chirpstack v4 can not process downlink message immediately in the queue when use Class C

Hi Team，I meet an issue when I try to send downlink message by the queue in the chirpstack v4. my end device works in Class C, I also select the Class C in the device profile, but I find the downlink message can not be processed immediately, it seems that only when uplink data is received, the downlink data in the queue can be sent. I also test the chirpstack v3，it works well, the downlink data in the queue can be processed immediately, is there any difference bettwen v3 and v4? Or I need other configurations in the chirpstack v4?

Have you checked whether all your systems are time synced? I had an issue similar to your symptoms described and figured that the downlinks were not scheduled because the chirpstack server and/or the gateway have not been properly time synced.