Why do my rails plain text emails sometimes work and sometimes don't

Question

I have erb and plain text versions of several emails. When I send them, some show the plain text version and some do not. There seems to be a bug in some of my plain text emails but not in others.

This is the plain text email that sends properly (app/views/user_mailer/send_first_welcome_everyone_email.text.erb):

Hi there,

Thanks for joining the TakeLessons community! 

We hope you enjoy using TakeLessons to advance your foundational music skills. 

5 - 10 minutes of practice every day will help you take your music skills to the next level.

Have a great day!
The TakeLessons Team

<%= render partial: 'vanilla_footer_text' %>

This is the plain text email that does not send properly (app/views/user_mailer/send_individual_day_reminder_email.text.erb):

Hi there, 

As requested, we are sending you this reminder to keep making progress on the educational games on TakeLessons.com.

Enjoy your music!
The TakeLessons Team

<%= render partial: 'vanilla_footer_text' %>

Is there something that would tell rails to only send html emails and not the plaintext that I would need to remove?

And the html versions of each of them are: app/views/user_mailer/send_first_welcome_everyone_email.html.erb app/views/user_mailer/send_individual_day_reminder_email.html.erb