If I fill my charcoal ring to the top with charcoal (regardless of lump or briquettes) and dump a half (or whole) chimney of lit on it (depending on my desired cooking temp), I'm gonna get more than four hours cooking time running in "BBQ range" (225 - 275*).
There has to be more to your story. I'll be happy to help as I know others here will as well. Please describe how you are set up from start to finish.