Hi Charles, Im sorry to hear that you are still in the same situation. If you are worried that your sensei may think you an aikido "slug" then I suppose you have to ask yourself a couple of questions:
A) Is my technique any good?
B) Is my attitude towards aikido/others a good one?
From what I have seen these seem to be the biggest reasons for not being tested, I know of people who were technically brilliant but had the wrong attitude and werent allowed to test untill they had corrected that. I also know of people who had a brilliant attitude but because their technique wasnt up to scratch were never considered for gradings.
If you know your technique is good and you know your attitude is decent then I dont see any reason why you shouldnt test. As others have suggested, only your sensei knows why he hasnt considered you for testing, maybe its time to have a chat with him?