Yes, it's the windscreen. You need some air to circulate and feed the flame, but not too much wind to blow the flame away from the pot bottom.
If you look at my windscreen, it should be a bit taller ideally. However, I wanted the screen to be able to nest inside the pot. It works well enough -- and the times when winds are really blowing, it's easy enough to mitigate by setting up in a protected area and by blocking some of the wind with your body, with larger rocks, etc.
I would say my stove boils 2 cups of water with 0.75 oz (three quarter full) fuel very consistently -- even when windy -- and I am very happy with it.
Some people really like Trail Designs 'caldera' system. I don't because I want my 'kitchen' to be both very light weight and very compact. And the caldera's are much too bulky when packed.
Pretty much all UL canister stove will work well with the SP600. Gigapower stoves and MSR PocketRocket are some that come to mind. With a narrower pot, just don't turn the valve all the way open as that makes for a wider (and more wasteful) flame.