Notification bug

[Muhammad Iqbal Pratama]

I am Zenfone 4 user, i'm having problem with the notification sound, when it's locked or "sleep mode", the notification sound is partially audible or nothing comes up at all when i receive a text message or IM or anything, call ringtone and alarm work just fine, i did some research through the net and nothing seems work, i'm tired of doing factory reset. Any suggestion?