Because certain verses (John 5:4, Matthew 17:21, Matthew 18:11, and Acts 8:37, among others) don't appear in some of the earliest and most reliable Bible manuscripts, some modern Bibles don't include them. If you come across a "missing" verse, check the footnotes—the translators have in some cases included the verses there or left a note explaining their omission.
If you come across a missing verse and believe that it's an error in our text (and not a translation decision), please let us know and we'll investigate it.