If a page contains more than 50 images, Exporting to Word will create a document with only 50 first images. All extra images will appear as empty boxes.
The number of images for exporting to Word is controlled by the system property atlassian.confluence.export.word.max.embedded.images.
The reason for the existence of such property is the stability of the system. Exporting is done in the same system process as the whole web application, therefore it is vital to limit the amount of memory consumed by the export.
Before proper fix is done, on versions above 5.2 it is possible to override system property with other value: