Check for recent changes you've made in the Mi fit or Notify and fitness app. reset anything to default regarding vibration. Try checking vibration settings in the 'Lab' section in mi fit.
If those don't work, try factory resetting the band, reinstalling the notify and fitness app for mi fit app, reinstall it and sign up with a new account.
(You can try shaking the band with your hand a couple of times. That may solve difficulties with the vibrating motor of the gears)