After you make sure the burner feed is clean of dirt
and cobwebs, the regulator is the next suspect.
Try to find a repair depot for your BBQ in your area
and talk to the mechanic. Tell him you heard about
some way to reset the regulator to allow full gas flow.
That's what I had to do. On mine, the method was,
just going from memory, shutting valves and tank off,
opening tank fully, then opening valves quickly.
Or was it the other way around? Can't remember now