Try each of the following steps in order until the issue is resolved: 

1. If the constituents do not have a gift, use a mailing function, such as Quick Letters, that does not require the constituents to have specific gifts.
2. In Records > Gifts, open gifts that are not included in the letters:
3. Mark the gifts as Not Acknowledged.
4. Ensure the Gift Date falls within the range of the mailing.
5. Add the appropriate letter code to the gift if applicable.
6. If filtering on Letter Code in the parameters, include all applicable letter codes in the Letters field on the Filters tab.
7. Ensure that the gifts are not being filtered out by any filters on the Filters, Attributes, or Addresses tabs.
8. Ensure "Exclude zero amount gifts" checkbox is not marked on Gift Types tab
9. If using a conditional mail merge in the Mail Merge Wizard, be sure to include all letters for gifts that are being processed:
   1. Be sure that the parameters are set up to create a conditional merge letter.
   2. Be sure the correct Conditional Field has been selected when going through the Mail Merge Wizard 
10. If the Donor Acknowledgement Letter includes a query, confirm the missing gifts are withing the query results