Why is my text not saving correctly in Acrobat Pro XI?

I use Adobe Acrobat XI Pro. When I use the add text function, everything shows up correctly. I save the file and close it. Later on, when I open the saved file, some of the text (especially capital letters and symbols) show up as little boxes with an X in them. I have to highlight, cut, and paste everything in order for the text to show back up. Once it happens one time, it usually does not happen again. However, if I open up a document, fix the boxes, add additional notes, those notes then turn into boxed Xs the next time I open the document.

How do I fix this?