Replace your flasher. Sometimes a good smack to the flasher will break a corrosion layer that can build up on the flashers' contacts if the truck sees infrequent use, and it might start working again. This has happened to me.

Clean up any grounds you can find at the signal lights bulb sockets, take a small wire brush and clean the bulb sockets. Check all bulbs. Finally, suspect the t.s. switch.

Steve