Service is still active:

systemctl -l status foo.service
● foo.service - My Foo service
   Loaded: loaded (/etc/systemd/system/foo.service; static; vendor preset: disabled)
   Active: activating (start) since Fri 2019-01-18 00:08:45 EST; 18h ago
 Main PID: 29032 (bash)
   CGroup: /system.slice/foo.service
           ├─ 4159 ssh: /root/....

This morning I was able to tail the logs with -f -u foo.service. But now I see this instead:

sudo journalctl -u foo.service
-- No entries --

sudo journalctl -f -u foo.service
-- Logs begin at Wed 2019-01-16 10:18:52 EST. --

I have another service (well, a timer to be exact) completed and log is also unavailable now.

How do I view old logs? I've tried using --since yesterday and _PID= for example, and it only returns to me system logs.