Was the feedback valid? I personally cant remember the rules because it has been a while since I looked at them, and honestly dont care to spend the time to locate them again because I am about to go to bed...but I know that there are rules as to what can and cant be left feedback for. If the feedback was not for a valid or qualifying "transaction" then it may have been removed for that reason.
If you are acquainted with the rules and still dont know why it was removed I would PM a moderator and ask. They may not be able to directly help you, but they can probably point you in the right direction. It is a good place to start IMHO.