You might try cutting the straw in half (or nearly in half) lengthwise and putting the open end against the "wall" of the cart. And also, don't overfill the cart. The straw mod makes it easier to fill, which makes it easier to overfill, and if the juice is saturating the polyfill all the way down, then when the atty starts pushing into it, there's nowhere for it to go except out into the atty.
If you're also dripping a couple drops on the atty bridge, you might eliminate that step and let the overflow from the cart take care of "priming" the atty. I used to drip on the bridge and then fill the cart, but now I fill the cart first, and if it looks like I did one drop too many (because it doesn't vanish down into the filler), I just put the cart straight on and turn it cart-side up to let the atty bridge take up the excess.