Missing Custom Activity Field Data When the Activity is Filed on a Case.
First check the log files (Where are the &quot;ConfigAndLog&quot; and &quot;templates_c&quot; directories?) and see if there's anything obvious happening there.

Otherwise if you can debug in CRM/Activity/BAO/Activity.php in the copyExtendedActivityData() function that should help track it down. That's the function that gets called from _convertToCaseActivity() in CRM/Case/Page/AJAX.php to copy the custom data and attachments.