Missing a lot of Downlink message


I’m using rak3172 with chirpstack server.
Device join successfully, only one device on the lorawan network, device stay very near to gateway. Internet speed is good. Message was sent every 30 second with confirmation. Region is US_01
At first 20 message, Downlink can receive but after 20 message, Rxtimeout happen a lot. It make Downlink counter only 50% of uplink counter

you can check below picture
Could you please help me to investigate further about this issue ?