This may occur for a few different reasons.

One reason is that the merge fields were copied from a different location and pasted into the current content. Merge fields should be unique to each location. For example, if you are working on a registration confirmation, you should add only registration confirmation merge fields that are available within the WYSIWYG editor. If you are working on a PDF receipt, only add merge fields available within this WYSIWYG editor.

Another reason the data may not populate is due to the conditional logic setup within the merge fields. This is especially true for dynamic content created within dynamic content merge fields or profile content merge fields. Review any Else statements for double negatives.

If the above do not apply, delete the current merge fields and add them manually using the WYSIWYG merge field icons.