Sometimes I do not see the Activities that are linked with a certain contact. Why is that?
The Activities tab is dynamically filled via a search query. Depending on the size of your message store and hard disk activity, this query could take a while to find all the results.
There is a status indicator at the right top with a "Stop" button. The button will become grayed out when the query has finished.
Corruptions in the pst-file could terminate the search query prematurely and not all or none of the results will be shown. In that case you should run scanpst.exe against your pst-file to check it for errors.