The open not showing up right away could be due to various factors, simplest of which could just be lagtime of the action actually showing up in reporting.
If this isn't the case I'd suggest going through this checklist:
https://community.marketo.com/MarketoResource?id=kA650000000GxXsCAK
Make sure you haven't deleted the tracking pixel from your email template, and that your email tracking links are set up properly - Marketo support should be able to help you troubleshoot this. Hope this helps!