My Reillo 3 RDB has locked out repeatedly?

On pressing lockout button, it appears to fire up and run for 10-30 seconds. It then stops, trys again a short while later, runs again, stops again. It repeats this process until it eventually locks out. I have removed red cover, then flexible fuel hose and fuel is flowing freely. I checked photocell and hat is clean. I removed whole unit and checked there is no build-up of soot.