PPCGeeks - New Text Message Notification Will Not Vibrate

Sorry Guys if this is already out there, but I can not find what is causing this and it's only started within the last couple of days.

Every time I receive a new text message and have left the last conversation open (using the threaded style text messaging) and that person sends me a reply text the phone will never vibrate (when I have the phone on vibrate), but if I close back to the message list and the person replies to my text message it will vibrate. It has only started to do this in the last couple of days and I'm not sure where to even begin. It's kind of an annoyance epically if i leave the conversation open and forget about it for a while.

Help!!!!