havn't really noticed til today but we seem to have this issue for a long time now (~ 6 months)...stats are fully unrealiable. Sometimes it works and an attachment get a few views counted but usually this stays at 0 (yes, considered the hourly cron-update), no matter how many people view it.
Any idea what might cause this very selective counting?
If the cron is working then maybe there is a problem with the xf_attachment_view table (which the cron uses to update views). That is a MEMORY table so it's volatile. If the db server is having uptime issues then that would result in views being lost. Keep an eye on that table.