Why Aren't All of My Fields Available in Form Builder?

Hello,

I am building a form, and I am having trouble including some of the fields I want.  

In the Form Builder, when I click to add a new field and try to search for it or select it in the "Select Marketo Field" dropdown box, it isn't listed.  I noticed that several fields that show they are mapped in Field Management are not populating in that Marketo Field Dropdown box. Thus, they aren't available options in my form.

Does anyone have any ideas about why this may be happening, or what I can do to fix it?

Thanks!