A family of Microsoft word processing software products for creating web, email, and print documents.
You haven't told us whether there is anything different about the records not being merged. If those records contain double-quote characters in any field, that could explain what you're seeing.
Having double quotes in any field in a mailmerge data source - even if that field isn't referenced in the mailmerge main document - is liable to result in an incomplete or corrupt output. You can have single quotes, however and, if you pair those single quotes, they will approximate the appearance of double quotes. That can be achieved via a simple Find/Replace in the data source.