You must be doing something wrong because you have the 2.73.405.5 ROM so it will let you use instant root etc.
Why it is not working I do not know.
Can you give me the exact details that pop up etc.
Please be more detailed because I need to rely on the information you give me in order to build a picture of what's going on.
Right now I am going in blind and I cannot advise you any further.
Try reading this. May be the instructions are more clear.
If the phone still can't be rooted after following the instructions (please try to do it at least twice) then at the top of the page there are other alternatives if the main one fails.
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