Why won't propane come out of my nearly-new propane tank?
Open the tank supply valve very slowly and make sure the weed burner valve is closed when you do it. I mean if you think you are opening it slowly you are still opening it too fast. Some of these newer tanks have a safety feature that cuts back on flow if high flow is detected.

You may also have a tank with a defective valve, if the above method does not work, take it and exchange it for a different tank at one of those places that will trade a filled tank for an empty.

It could also be your weed burner valve, a lot of those weed burners are lame Chinese pieces of crap. Try it on an alternate appliance like a camp stove or BBQ.