Why are some of my contacts receiving plain text versions of my campaigns?

There could be many reasons for this. The first thing to check is the delivery format of the contact and/or group. It should be set to HTML.

Second, the contact's individual device (e.g. computer, mobile phone, etc.) where they are receiving the mail and the email client they are using to read the mail must be configured properly to receive HTML messages. Certain mobile devices, for instance, may only display the plain text version.