If you don't tick the 'www' checkbox when creating a domain in Plesk, the CNAME record for www.{domain}.{tld} is not created.
However, simply changing the setting or creating the CNAME record is not enough to get it working - Apache config needs rebuilding.
So to fix the situation, follow these steps:
STEP 1
EITHER:
Change domain settings:
Plesk > Domain > {domain}.{tld} > Domain Administrator Access
Domain name: Check the 'WWW' box
[OK]
OR:
Create CNAME record:
Plesk > Domain > {domain}.{tld} > DNS Settings > Add Record
Record Type: CNAME
Enter domain name: www
Enter IP address: {Main IP address for domain}
[OK]
STEP 2
Rebuild Apache config:
Login to server via SSH as 'root'
OR