How do you get soda siphon water to be fizzy?

I bought one of those Is2 Soda Siphon bottles, you fill it up with water and then add a CO2 charger. WHen I follow the instructions exactly, and make sure that the water is cold, the resulting soda is quite flat. When I fill up a glass it is very bubbly on top (the foam) but the actual liquid is flat.

Any suggestions?

Are you giving it a good shake? I found that doing this (straight after the CO2 charge) was a vital step. Also don’t overfill it with water. There should be a mark - on mine it was 2/3 of the way up the bottle. Putting more water in made it less fizzy.

After some research it looks like the iSi siphon, that I bought (and is sold at Amazon), has a very low fizz due to some bizarre ‘safety’ mechanism that limits the co2. Consumer reports has an online article where they review the siphons, and iSi scored the lowest. (Found out too late)