[scribus] Help with "placed pdf"

[AnnaMaria Hedin]

Am working on a music book project that needs importing pdf-fils with 
the music scores into picture frames.
Tha book is 80 pages or so, rather big document.
It looks nice on screen and prints nicely.
But I need to make it to a PDF file in order to send it to print 
proffessionally and make it into a book.

That doesn't work.
Some of the pdf files disappear - I get a prewiew report that reports 
"problem":
it is alert with yellow buttons (red buttons if a picture is missing) 
and say
the "picture is a placed pdf"
but I get no explanation
WHY that is a problem
or
WHAT I can do to fix it.

Suggestions for a solution, PLEASE!