Hello Amy,
When we send an Action Item, we also generate an email to the user with the link to the Action Item. If in case the Action Item does not get approved or rejected within the necessary timeline, it automatically gets rejected by the process and the Action Item is no longer present within the My Action Items Portlet. You may send an email notification to the owner of the Action Item as well when the Action Item goes to the user, similarly when the user approves or rejects the Action Item. And, if in case the Action Item is not approved or rejected within the relevant timeline and finally, gets automatically rejected, the owner of the Action Item gets an email notification again about this. So, this will help the owner to know if the Action Item was approved or rejected by the user or gets automatically rejected by the process. We have not really sent reminders for pending Action Items, but, you can see if this is really required as there already will be a previous Action Item that has still not been attended by the user and can create a confusion. May be an email reminder with the link to the Action Item would help.
Suhail.