My propane burner is hard to ignite, and when it does, the flames are small

I have a Bayou Classic KAB4 propane burner, which I've had intermittent success using to bring my wort to a boil. When I hook it up to a full tank it seems to work fine, but when I use a tank that is slightly less than full, it takes several tries for me to ignite it, and when it does, the flames are extremely small - even with careful adjustments to the fuel and air intake valves, the they die quickly. Just to make sure I'm not going completely nuts, when this happens I hook the tank up to my propane grill, and lo and behold, it works perfectly with my grill.

I do notice that when my burner is hooked up to a full tank, I can hear the characteristic propane hiss when I open the valve. With a tank that's mostly full, this hiss is much more faint. (With my grill, the hiss is the same no matter how full the tank is.)

There are no leaks in the hose. I even took the burner into the homebrewing store where I bought it and the guys there couldn't find anything wrong with it. My workaround has been to swap out my propane tank for a full one every time I brew, which has been taking a steady toll on my wallet.

Not sure if this makes any difference, but I brew in an open garage with both the burner and the tank on the ground, roughly at the same elevation. I've seen other brewing setups with the burner elevated. Would elevating the burner above the tank make a difference?

Is there anything else I should check for in my burner, or do I need to modify my setup in some way so my burner will behave more consistently?