Make sure all your fittigns are tight. If air is getting in anywhere, you'll lose suction. Also be sure the hose isn't kinked or being crushed anywhere.
Beyond that, you may not have enough pressure to create good suction. I have a faucet in my house where it works very well, and another where it doesn't.
Sometimes I just open a window and start a siphon without the it being on the faucet to save water anyway.
It is a violation of federal law to use this product in a manner inconsistent with its labelling.