Marketo considers an "open" to be when the images in the email are downloaded, specifically a single-pixel tracking image. The recipient may be receiving the emails and viewing them, but not downloading the images. This would not count as an "open." Please note that since text-only emails have no images, so they will never log an Open activity.
If the images are being downloaded, and you are still not seeing the activity after about 5 minutes, check to make sure there are no duplicate leads that are logging the activity instead. If you need further assistance, please contact Marketo Support.