How can I get my Sime Friendly Format 80E to stay on when we need it to?

This boiler is in our cafe. We use it to heat water and radiators. We do not have a manual for it. The problem seems to relate to the timer. The boiler fires for a short time then goes off. We have tried "playing" with the timer but cannot get it to stay on for the period of time we need it. can you help please?