Templates derived from the Document template are not "compatible" with templates derived from the Text template, therefore the Template dropdown for a Text derived item will only list the templates that are compatible (also derived from the Text template), and vice versa.
Note that the Appointment, Contact and Task templates (and their derivatives) behave somewhat differently (they can "cross-over") because they can either store an Outlook object or can function like a Text item (store rtf text).
It sounds like this is what you are seeing. If not, please email a sample .urd file demonstrating what you are seeing to
support@kinook.com and we will investigate further.