Workflow notification email error for Purchase requision release

Hi Friends,

two users are facing problem to execute workitems from outlook inbox for purchase requision.

There are 2 SAP icons in Outllok inbox.

1. It will take the user to SAP inbox.

2. It will take the user to excute the item.

If the user is not logged into SAP and clicking on Second SAP Icon it is taking to SAP inbox but not to execute the workitem.

If the user is logged into SAP and clicking on Second SAP Icon it is taking to execute the workitem which is correct.

This is happenning occassionally but not always.Please let me know what will be the reason to happen like this?

Thanks&Regards

Bhuvana