Downlink data sometimes not transmitting immediately (unconfirmed) for class C device

Hi All,
I am sending unconfirmed downlink to my Class C device But sometimes the server is adding downlink to Downlink queue instead of send the package.
Without any detected reason.

Please tell me how resolve this problem

Currently Using -

chirpstack-application-server version - 3.17
chirpstack-network-server version - 3.16
chirpstack-gateway-bridge version - 3.14

Device profile configuration - Class C, LoRaWAN MAC version - 1.0.3, LoRaWAN Regional Parameters revision - A

I test it with 2 gateways, one in US915 and one in EU868, and see this behavior on both.

Package sent on 11:01 and Chirpstack hold for 4minutes.