Under pressure, my system was generating lots of messages like this:
# systemctl -l status systemd-journald.service Jun 24 13:47:23 localhost systemd-journal: Suppressed 15460 messages from /system.slice/...
This means the system is generating lots of messages and journal is configured to drop some of them. This is called rate limit, and is useful to not overload the logging system.
Sometimes, however, you just want to get all messages, or perhaps increase these limits. This can be achieved by setting the variables RateLimitInterval and RateLimitBurst inside the config file /etc/systemd/journald.conf.
Quoting the man page for journal: To turn off any kind of rate limiting, set either value to 0.