Still Coughing..

I brought my almost 2year old cat to the vet over a week ago because he has been coughing alot, sounds like a hairball cough but no hairball comes up ever. The vet listened to him, and didnt hear any wheezing but said his heart rate seemed high but that it could of been due to him being so nervous at the vet. He prescriped antibiotics Clavamox to be exact for a week to see if it would help, and said that if it didnt he would recommend a chest x ray. Welll last thursday was 7days on the meds, and the cough hasnt responded to the meds. Hes still coughing at least two or three times a day that I hear. Im going to call the vet tomorrow AM to get him in for an xray... What could be going on????