This may be happening because of the layout options of the text boxes. If the text wrapping option of the text box is set to behind text Altru will not recognize it. Try the steps below to fix the issue:
  1. Click on the layout options icon of the text box
  2. Change the text wrapping to the "tight" option
  3. Save the letter
  4. Re-upload the letter into Altru
  5. Perform the merge again
This issue may also happen with other options other than the "behind text" option