Symptoms
When configuring out of office for users it is not working. The work items are not being forwarded but still going to the original user only. You may observe this issue with out of office both from the K2 workspace and from the worklist.
Diagnoses
Basic thing to check first (and this is being overlooked quite often) is work list filter settings for your destination user - it can be customized and consequently this user just doesn't see these tasks because of filter settings. There is an icon with a wrench in the top right corner of worklist which will allow you to see/adjust filtering settings.
If filter is not an issue it is worth trying to force identity refresh for both users (original and delegate). Once this is done for both users please retest out of office functionality. If it still does not work check K2 host server logs.
There is also a number of other possibilities here. Please refer to the other community KBs:
Out of office does not forward assign task to specified user
http://community.k2.com/t5/K2-blackpearl/Out-of-office-does-not-forward-assign-task-to-specified-user/ta-p/86460
Out of office configuration
http://community.k2.com/t5/K2-blackpearl/Out-of-office-configuration/ta-p/83265
Resolution
See above.