Merge fields come out blank or show as {{...}}

The two symptoms have different causes, so start by looking at which one you actually have.

The tag shows literally, as {{customer.company_name}}

The placeholder was not recognised at all. Either:

• The field name is wrong. A tag only resolves on an exact match, so a guessed or mistyped name stays as text. Delete it and insert the field from the picker instead of typing it.
• That record type is not part of this send. An {{opportunity.…}} tag sent from a customer with no opportunity attached has nothing to resolve against, so it is left alone. The same applies to {{contact.…}} when the message is going to a typed-in email address rather than to a real contact.

The tag disappears and leaves a blank

This is the more confusing one, and it means something quite specific: the tag was recognised and resolved correctly, and the value it found was empty.

In other words the template is fine. The field is simply not filled in on the record you tested against. A template testing beautifully on one customer and leaving holes on another is the normal way this shows up.

To confirm it, open the record you tested with and look at that field. If it is empty there, that is your answer.

How to fix it properly. Fill in the field on the records that matter — adding it as a column on the list page shows you at a glance which records are missing it, so you can spot the gaps without opening each one. If a field is routinely empty, either it is not worth merging into customer-facing email, or it needs to become required when records are created.

Always preview against a real record before sending, and ideally against an awkward one rather than your best-maintained customer.