When I preview my forms some textboxes are missing.

Also, I can see all the fields when I'm in the builder, but when I preview my forms some textboxes are missing (every single one of the "chèque n°" labelled textboxes are gone).

This is due to conditional statements setup on the form, you can delete or disable them:

Here is a guide that might be of your interest: http://www.jotform.com/help/57-Smart-Forms-Using-Conditional-Logic

Let us know if you need more help on this, and if you have a different question please open a new thread, we will be glad to assist you.