My alarm just vibrate after a few weeks. No sound just vibrate

I Have an iphone 6s and after a few months of using it, i noticed that my alarm is not sounding off. I will fe-boot and for a few weeks it will go off then just vibrate. I need a reminder everyday at the same time so i set the alarm at a certain time on my calendar. Is it possible to get an exchange of phone? My carrier said it is a phone problem not a network one. Can you please help me? I went to an apple store at the queens center mall,and all they did was reset my phone. It helped for a few weeks then same problem. Help! Thank you for your consideration.