Stock bulbs or Leds?

assuming normal bulbs, first check none have blown, then are they in the holders correctly? any corrosion? then check the relay at the back, are the connections good and clean? how is the switch gear? check clean the inside it can get crummy in there and for some reason spiders seem to love it in there too.

if LEDs do you have the right flasher relay installed? or resistors? are all your connections good?