Wrap a towel around your bass neck to protect the finish. Take a toothpick and put 1-2 drops of WD40 on it. Apply carefully to truss rod nut (the WD40 could mar your finish, hence the towel). Let it sit for a couple of days and then try again. Also, try loosening your strings. Sometime the tension of the strings can make it difficult to move the truss rod. I've had this happen even when trying to loosen the truss rod.
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