You have been sending live emails (non-samples) to yourself or colleagues to test functionality of an upcoming email blast, and even though you/they may be opening the email, you are not seeing the open email activity on the Lead Activity Log.
Note that there may be a slight delay between opening the email, and the activity displaying on the Log.
Marketo considers an "open" to be when the images in the email are downloaded. The recipient may be receiving the emails and viewing them, but not downloading the images. This would not count as an "open."
If the images are being downloaded, and you are still not seeing the activity after about 5 minutes, there could be another issue that Marketo Support could investigate into.