Help! New Grill not flaming properly!

Question

Having a problem with a low flame. It worked fine for about 5 or 6 grillings, then I cleaned it and now it flames up but nowhere near what it should be when on high. Had the plumber that hooked up my natural gas converter come back and take a look- couldn't find anything wrong with hook up or gas pressure. He suggested cleaning out the carryovers, etc. (which I did even though I've only used it a few times) but it still is not reaching proper high flaming levels. Any suggestions?!! I have a barbecue in 5 days and I'm in a panic! Thanks!