Sometimes, there are just too many messages in journalctl output and it becomes a mystery game to search for the messages you are looking for. But luckily you do not need to use grep to find the right message. Here is example of what I had to do when I was looking for kernel messages.
# To see all the fields, you can use the verbose mode
journalctl _TRANSPORT=kernel -o verbose
# And the filter on priority if needed to get the messages you need
journalctl _TRANSPORT=kernel PRIORITY=4
# and follow
journalctl _TRANSPORT=kernel PRIORITY=4 -f -l