(Symptom - certain agents do not get queue calls - has multiple possible root causes)
If certain agents do not often get queue calls (or never get queue calls), you may have mis-configured the routing priority for your queue.
To check agent priority, follow these steps:
Click on ACD --> view queues
Click on the name of the queue you wish to modify
Under the 'Priority' heading within the 'Add Queue: agents' category, make sure that all agents are set to priority => normal unless you specifically want certain ACD Agents to handle a majority of your queue callers.
Click 'Apply All Changes' at the bottom of the page