Hi,

Have you logged into more then one Apple ID’s if so than that is your problem, find the info for the other Apple ID and remove the iPhone from that Apple ID and the one you have been using at https://appleid.apple.com/#!&page=signin

For the correct account the phone will show in the Find iPhone section of it on http://www.icloud.com

If you bought the phone new (i.e. it’s not second-hand), you could try going to an Apple Store (make a Genius Bar appointment if you do) or try contacting iTunes Support online (https://www.apple.com/emea/support/itunes/contact.html) and see if they can remove the phone from the account - they are likely to want to see proof of ownership before they might help e.g. original receipt with the serial number on it