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. Once the query has finished, the button will become grayed out and the search icon will stop spinning.
The Activities’ search icon sometimes spins forever before it produces some results.
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.