This might be related to a lack of support for specific image encodings. I have PDF files created by other software which show missing images only in PDFArchitect, images that were imported as original PNGs with limited palette or with transparency. One missing feature may be a JBIG2 decoder (here the Foxit Reader tried to install a plugin), another may be handling of transparency layers (Foxit Reader could display these images, PDFArchitect not).