Message API: Failed to send messages

Hi,

I got a lot of responses "400 Failed to send messages" after running the loop as the psudo code below...

for ( 900 times ){
    curl( POST https://api.line.me/v2/bot/message/push ) with valid GroupId
    sleep 0.2s;
}

80% of responses has httpcode 200. Rest of them has httpcode 400 with message "Failed to send messages".

According to LINE API Document

** Failed to send messages**: Appears when the message fails to be sent. One reason this may appear is if the user ID specified does not exist.

But, I am sure each GroupId is valid and can be used to send messages successfully in another test. Does someone have similar situation like that and know how to solve it?

ps. I'm using a paid-plan Message API.