If it's all text like Erik said, Opens are tracked by links / Images loading / called actions. Not them clicking to open your email. A good way to prevent this from happening is throw your logo in your email somewhere or even a super small transparent PNG in the footer (or something) but I would recommend your logo 🙂
If you have images in your email but still aren’t seeing anything. You may just need to wait a little bit. Or contact Marketo Support.
Litmus, Google Analytics are both great for tracking email as well. Just a thought 🙂