Unreliable systemd user service

[Aidan Gauland <aidalgol@fastmail.net>]

I have a user service for running xautolock that does not start on login
reliably, and I have no idea why, because there is no error message,
just an exit code of 1.  (Unit file and output of systemctl status
attached.)  Any suggestions on what to do next to troubleshoot this?

Regards,
Aidan Gauland

● auto-screen-lock.service - Lock the screen automatically after a timeout
   Loaded: loaded (/home/aidan/.config/systemd/user/auto-screen-lock.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Sun 2019-06-16 16:29:26 NZST; 43min ago
  Process: 1542 ExecStart=/usr/bin/xautolock -time 5 -locker /home/aidan/bin/i3lock (code=exited, status=1/FAILURE)
 Main PID: 1542 (code=exited, status=1/FAILURE)

[Unit]
Description=Lock the screen automatically after a timeout

[Service]
Environment=DISPLAY=:0
ExecStart=/usr/bin/xautolock -time 5 -locker /home/aidan/bin/i3lock

[Install]
WantedBy=default.target