The banks are telling me that the Durable Power of Attorney, written up as part of a trust, can't be used on accounts in a trust. Why is this?

I dont know about the legalities of what you are trying to do but I have personally found that the bigger the bank, the less likely they are willing to work with you. I have a POA for my Navy Son and it seems like the big bank in town just doesnt want to take the time to honor the POA. Our smaller one goes out of their way to work with me.