Volume's turned up & it's definitely not on silent mode but the default alarm isn't going off, it's just vibrating. iPhone 4 with the newest version of iOS (I

Volume's turned up & it's definitely not on silent mode but the default alarm isn't going off, it's just vibrating. iPhone 4 with the newest version of iOS (I think). Edit: it goes off if the app is closed, if it's open it doesn't.