Not receiving Facebook message

[artdaniel306]

I'm using Ubuntu 20.04 to run Chatwoot.
Self hosted env
OS: Ubuntu 20.04

I have setup a Facebook inbox in Chatwoot. When I send message from Chatwoot, I can see the message on Facebook page immediately.
However, it usually takes much longer time to receive message from Facebook. Usually 30-60 mins. Sometimes, no message is delivered from Facebook.

According to Chatwoot FB setup. I have turned on the following Advanced Access in Facebook App and passed the verification process:
pages_messaging, Business Asset User Profile Access, pages_manage_metadata, email, public_profile

FB APP has registered the following webhooks:
inbox_labels, message_deliveries, message_echoes, message_reactions, message_reads, message, messaging_account_linking, messaging_handovers, messaging_optins, messaging_policy_enforcement, messaging_referrals

nginx.conf

user www-data;
worker_processes 4;
pid /run/nginx.pid;
include /etc/nginx/modules-enabled/*.conf;

set open fd limit

worker_rlimit_nofile 30000;

events {
worker_connections 10000;
# multi_accept on;
}

http {

    ##
    # Basic Settings
    ##

    sendfile on;
    tcp_nopush on;
    tcp_nodelay on;
    keepalive_timeout 65;
    types_hash_max_size 2048;
    # server_tokens off;

    # server_names_hash_bucket_size 64;
    # server_name_in_redirect off;

    include /etc/nginx/mime.types;
    default_type application/octet-stream;

    ##
    # SSL Settings
    ##

    ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3; # Dropping SSLv3, ref: POODLE
    ssl_prefer_server_ciphers on;

    ##
    # Logging Settings
    ##

    access_log /var/log/nginx/access.log;
    error_log /var/log/nginx/error.log;

    ##
    # Gzip Settings
    ##

    gzip on;

    # gzip_vary on;
    # gzip_proxied any;
    # gzip_comp_level 6;
    # gzip_buffers 16 8k;
    # gzip_http_version 1.1;
    # gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;

    ##
    # Virtual Host Configs
    ##

    include /etc/nginx/conf.d/*.conf;
    include /etc/nginx/sites-enabled/*;

}

Wonder if I have missed any setups or configuration. I have googled but no luck finding relevant posts regarding long delay problem from Facebook.
Thanks.