There maybe a few reasons for Markups missing. The first is you have to look under the correct version of the drawing in the Markups tab.
The second is probably the user trying to view the markups is using an older DWF viewer any viewer that is not DWF Composer or Design Review will view the DWF but without the markups.
I would check in the preferences section in TOOLS and see if DWF files are using "Buzzsaw Viewing Controls" to view. If they are using VoloView or some other older viewer the markups will not show.