Plexamp isn’t updated as frequently as every week. You can instead check for updates on the 1st day of every month with the following:

# Upgrade plexamp every Monday early AM
0 3 1 * * /home/USERNAME/plexamp/upgrade.sh
30 3 1 * * sudo /usr/sbin/reboot
# Fix the username used in the service file
35 3 1 * * sed -i 's/User=pi/User=USERNAME/g' plexamp/plexamp.service && sed -i 's/\/pi\//\/USERNAME\//g' plexamp/plexamp.service && sudo /usr/sbin/reboot