Make sure all the bulbs are burning when in the flasher mode. This is a classic symptom of a bulb burned out.

And if the bulb checks good check the socket for electrical connection. The ground, which is the bulb holder, is a known area to fail. But it is easy to fix.

You just need to hit it with a bit of sandpaper, both the inside of the socket and the metal body of the bulb, and when proper contact is made the flashing should stop.

That is, if this is the trouble.

The positive side of the socket, which is the small metal disc in the center of each socket, can also get loose. I have found a bit of aluminum foil stuck in there is all it takes to make the positive contact high enough to once again make contact with the bulb.