"Lazy" Yellow Flame Atwood Water Heater

Could just be low gas pressure/air in the line...first thing I'd try is to turn off propane at tank, then turn it back on very slowly until valve is all the way open. Then go inside and light a stovetop burner, letting it burn long enough to insure that there's a full flow of gas. Then try lighting W/H again.

If that doesn't do any good:

Scroll down the Atwood gas/electric W/H page at this link and you'll see a section marked "Erratic Burner Flame or sooting". It gives a number of other possible causes.