The individual ack mode is not supported for durables at the moment. There is a fix in 5.6 to flag this as an error.
The problem is that the store just keeps track of the latest acked sequence number and has a shared message store. To support individual ack mode, would require backing each durable with a queue.
You could look at using Virtual topics in place of Durables, in this case, consumers subscribe to a queue and the virtual topic dispatches to each of the subscription queues.