Sometimes you will noticed that images contained in the original PDF document were not optimized and remained as is in the output optimized PDF document.

Most likely these images are inline images and Qoppa’s jPDFOptimizer library will not optimize inline images. The reason for this is found in the definition and usage recommended for inline images in the PDF specifications:
Inline images are images that are defined directly within the content stream in which it will be painted rather than as a separate object. Because the inline format gives the application less flexibility in managing the image data, it should be used only for small images (4 KB or less).