The form part of the PDF is a FormsCentral form that you inserted into another PDF. The effect of that is the "PDF Producer" is Acrobat and not FormsCentral as it would be for a FormsCentral PDF form. The import PDF process processes FormsCentral PDFs differently to avoid this type of issue but since the PDF Producer changed the import can't do the right thing.
There are two ways to go about this:
1) The best method is to distribute the original form from FormsCentral as a submission enabled PDF and then insert the other content pages into that PDF (or as you did hered insert the form into the other PDF). You can edit the FormsCentral PDF so long as you are careful not to "break" the submit by editing the fields. This FAQ helps with editing a FormsCentral PDF:
By doing it this way you would still be able to edit the form in FormsCentral and save a new Submissio enabled PDF anytime to pick up the changes and it would not change where the responses are submitted to (all responses would still come into the same form in FormsCentral), also you would have a web URL available to distribute if you desired (maybe embed into a web page that includes the prepending pages in your PDF if you wanted one day).
a) go back to FormsCentral and open the form you had created
b) from the Distribute tab save as PDF with the "Add Submit button and have responses sent to FormsCentral" checked (Note: you will need to be signed in and if the form is not online move the form online from the "Collect Responses Online" tab)
c) open the newly saved PDF from FormsCentral and "Save a Copy" (per the FAQ above)
d) open the copy you created
e) edit the PDF choosing "Pages" and Insert the additional pages you wanted before the 1st page of the form
f) following the instructions in the FAQ Reader enable the PDF and it is now ready to distribute to your users
2) Another option, although I recommend #1, is to edit the FormsCentral PDF instead of the "other" PDF and insert pages into the FormsCentral form rather than the other way around, this would not change the "PDF Producer" from being FormsCentral (a property of the document that we look for on import) and you could import that updated PDF into FormsCentral. Again here, if you wanted to do it this way see the FAQ above about editing a FormsCentral PDF in Acrobat.
By doing it this way you would lose the ability to edit the form in the future and continue to collect the responses in the same form in FormsCentral, you also do not get a web URL for an imported PDF form.