I had some error messages not appearing in simple_form because the fields they were related to did not appear in the simple_form. Normally, simple_form will highlight the field in the form that has a problem and display the error message next to it. However, since the field related to the error wasn’t in the form, there was no message being shown.
To solve this, I had to add some additional code to display the text of the error message.
I fixed it by adding this to my simple_form forms:
<%= render 'shared/error_messages', object: f.object %>
and then created a partial, _error_messages (shared/_error_messages):
<% if object.errors.any? %>
<% end %>