Do all the lamps fail at the same time or just individual ones?
On my old 4/4 the rear nearside was a bit hit or miss due to a poor earth in the bulb holder. All other indicators were ok.
As for the flasher unit itself, take it out and clean the contacts. Check out this site too for a bit more info.
https://www.howacarworks.com/accessories/testing-and-replacing-a-flasher-unit
