Why doesn't our email template merge the fields from a related list?
|Knowledge Article Number||000003303|
The problem here is that related child information cannot be pulled into an email template. Multiple records can be in that related list and the system can't tell what record you want it to pull into the email template, it's also not possible for the email template to pull ALL of the records. In order to pull information into an email template there must be a lookup field to that record, much like Contacts lookup to Accounts. The system sees one single record named in that lookup field and pulls that information down.
In the end there's not a lot that can be done in this situation, other than to work around the standard Salesforce functionality for a one to many relationship situation.
Another possibility is to try to change the process and create lookups to the custom object instead of, or in addition to, using the related list. You will need one lookup for each custom object record you want to pull into the email template. This is not advisable if there will be many records, for instance, if you expect that a contact could have 20 or more orders you would have to create that many custom lookup fields. EG. Order 1, Order 2, Order 3, etc.