Two emails we've sent out are not recording open rates correctly. It seems like the reports are registering only subscribers who Open AND click a link. We've confirmed that open emails (without clicks) are not showing up in reports. Anyone else experienced this?
We can only record an open if the images are downloaded (particularly the tracking image) - see http://help.campaignmonitor.com/topic.aspx?t=89 for more on that.
So if images are not showing, or some kind of tracking image prevention software (this does exist) is in action, we can't detect the open. Clicks in the HTML version are always tracked though, and that obviously means the email has been opened too.