Why are some form fields disappearing while moving through the form pages?

I have Back and Next buttons on my form which cannot be seen in design mode only in preview.

If I use the back button when running the form it seems to skip a big part of the form and that part goes missing until I restart the form. Why?

I have been testing your form; however, I'm unable to replicate the same issue, here is a screen-cast about the result of my test: 

May you please kindly confirm the issue does not happen with your form anymore? 

If the problem persists, please kindly let us know if this happens when you select an specific option on a form field or after performing an specific action on the form. 

We will wait for your response. 

Hi,
Please see link below. This uses jotform in an Web APP. Use the left hand button (The right one enables people to go back to an incomplete form).
You will then see the use of Next and Back buttons.
The Back Button is the issue as it does not go back as expected and pages get skipped and go missing???
www.eunisure.co.uk/cna-page
Stephen Bradshaw, IT Support
Office: 01638 722999
Mobile: 07876192324
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You have many too conditions, which hide pages. Perhaps, this is causing the problem. I would recommend you to delete all conditions, which hide/show pages and test again.

You can also try to remove the back button. In this way, the form will go in one way and follow its logic.