You must provide a new site-defined trigger for your custom field to trigger activity based on updates. The syntax is as follows: This file has a command of the type,
POST_CI val_fieldupdate_site(persistent_id, audit_userid, CHANGED_ONLY, zattr) 111 FILTER( EVENT("UPDATE") );
where zattr is your custom object. if you have more than one custom object, add them all separated by commas.
This trigger is entered via Web Screen Painter's Schema Designer for table cr (Request), within Site-Defined Triggers under the Advanced tab.
If you examine the file cr_site.mod under site\mods in your CA SDM installation, the comments will explain this in more detail. It will also show that this must be done for each custom object you wish to perform Activity logging on.