Hi @MCarmen Cuadrado ,
If you are referring to The Scheduler app (I'm from the team developing this app), then to add new field to template, you just need to add it to the create issues screen.
In other words app will show you the same fields as those available while creating Jira issue.
In case of any troubles please us our app dedicated suport: Customer Portal
Please check if you are checking the same issue type.
If those missing fields are from 3rd party apps, then please contact our support with info from which app are those fields and how those fields are configured. Our team will try to reproduce it and guid you to solution (or we will do our best to add compatibility for this field)
Also please mention this ticket :)