Digital signature does not show up when inserted in PDF portfolio

I certified with visible signature a drawing and saved it to a separate file in Adobe Acrobat X. Once I open the file, the signature shows up correctly.
 
I follow the same procedure with the rest of the sheets.
 
I assemble all the sheets together in a PDF portfolio.
 
When I open any one of the drawings from the portfolio, there is light blue shading over the area where my signature should be, but no signature is displayed.
 
When I print this document, it does print out with the signature as well. Basically, I just cannot see it on the computer screen.
 
So, if I open the file separately before I assemble the PDF portfolio, it shows the signature. After I put it in the portfolio, it prints the signature but it shows the blue shading in its place on the computer screen.
 
Any advice, help?
 
Thanks.