Normally the main control over the propane is the thermocouple. It ensured the gas is turned off if there is no heat, so that you don't get an explosive build up of gas.
But I doubt this is the problem because it would not likely come and go, or be weak.
So my best guess is the orifix jet is getting clogged.
The opening the gase comes out is small, and is can get clogged easily from dirt in the line before you connected it. Water and sediment can also build up in the tank. There may even be a filter on the device that could be clogged, but most do not have a filter. Instead you need to remove the final jet orifice, see if it looks clogged or open, and blow it out. You can back blow the entire line if you take it off at the tank to let the dirt fly out. Careful if you want to stick a pin in to unclog it, because you don't want to increase the opening size.
As a final warning, if you run this in an enclosed space, it could use up all the oxygen, and that can also make the flame weak and go out. So make sure it has plenty of ventilation. Aim the exhaust out an open door if you can.