Propane, right? Sometimes it's as easy a disconnecting the bottle from the pit, and reconnecting it. Try that two ways: 1) With the bottle off, 2) With the bottle on
Sometimes shaking the bottle makes a difference, try that and then the above. Another thing is that the bottle might not have been bled before refilling, and there's too much air in the tank to get a good fuel run. Try connecting a different bottle
If that doesn't work, then like Steve says, a good cleaning. A lot of times, spiders get in the manifold and build nests, clogging up the manifold. So maybe just getting some pressurized air and spraying into the manifold fixes the problem.
Those have been my experiences, yours may vary