Motorola V710

When I put an event in the calender and have a reminder set for it and my PHONE IS ON VIBRATE, sometimes it lets me know and works fine and other times it doesn't. I have no idea why this is. when it doesn't work right, sometimes it shows up on the screen but doesn't vibrate and other times it doesn't show up at all. I am on my second phone with this and the same thing happens with my friend who has a v710 too.

anyone heard of this? help thx