Default Phone stops vibrating for text messages

It happened when I had Windows Mobile 5, 6 and now 6.5. It would vibrate for a while for whenever I received a text message, then it would just stop out of the blue. I would go to the notifications screen and vibrate is still checked for text messages. I would turn it off, then turn it back on, and the phone would vibrate for about 4 or 5 text messages then just stop again. Is there anything to do?