There's a little temperature probe that is positioned such that its right in the flame, it's called a thermocouple. It is connected via a small tube to the gas control valve. When the thermocouple reaches a certain temperature it pushes a contact in the gas valve telling it the flame is burning and not just pumping the trailer full of propane, and that it's ok to let the propane flow.
After they've sat dormant for a while a thermocouple often becomes weak or 'fussy' and seems to want more heat than usual. Often you can fool it by heating it with a small propane torch. Sometimes after a few cycles like this they start working, sometimes they need to be replaced. They're not much money.
Ted 'n' Laurie, plus Jackson (aka Deputy Dog, the Parson Russell Terrier 'fur kid') and, Rylie (who crossed the Rainbow Bridge June 14, 2012).