Aim higher than normal. The can should be slightly below the scooping crosshair. The front of the Python seems to smack the cans before they can be slurped.
I found it dead easy in the Cobra, but a lot more difficult in the Adder. Kept knocking it away. Now I keep it in the bottom half of the scanner, and keep it slow, works a bit better but still not as easy as in the Cobra.