The best thing to do is log to a remote log server.
If adjusting the number of log entries visible using the filter in that view is insufficient, you can use this command to save all IPsec logs:
clog /var/log/ipsec.log > /tmp/ipsec.log.txt
Execute that in Diagnostics > System Command
Then, on that same page, Download File /tmp/ipsec.log.txt
The logs kept on the firewall are circular, however, meaning old entries are overwritten by newer entries. The amount of logging kept is set in Status > System Logs, Settings, Log file size (Bytes). What you can do there depends on your disk size. I have mine set to 50000000 (50MB) on a system with a 30GB mSATA and it is still 90% free (about 3GB used Disk space currently used by log files is: 1.2G Remaining disk space for log files: 22G). You have to reset all logs further down on that page for this to take effect.
You can save a lot of the system state in a status output file. That is taken by navigating to https://firewall.address/status.php and downoading the resulting file. On busy firewalls that might take a moment to run. And for IPsec issues the logs saved there are often insufficient so the status output should be coupled with an ipsec.log.txt file as described above.
If you have more than one tunnel it is often beneficial to get the conXXXX number of the tunnel from ipsec statusall so you can filter on it (and filter out other tunnel logs) using grep, etc.