I'm Doug, and I'm very sorry to hear of your situation. My goal is to provide you with excellent service today.
Unfortunately, while a duly executed POA gives you the right to act on your mother's behalf, it is not a document that others must legally comply with. They may comply if they wish and be protected because you are acting as your mother's agent---however, no business or person is legally obligated to deal with you.
The agent is likely trying to eliminate as much work for himself as possible by refusing to deal with you.
I would suggest that you draft a simple letter from your mother to the agent, directing the agent to deal with you on any and all aspects of the pending listing and sale of the property, and if the agent will not comply with the direct demands of your mother in this respect, consider replacing the agent for his failure to comply with the reasonable demands of his client.
You may reply back to me using the Reply link and I will be happy to continue to assist you until I am able to address your concerns, to your satisfaction.
Kindly, remember to rate my service to you. That is how I am credited for assisting you.
I wish you and yours the best in 2015,