I made an ISI Whip recipe for Bell Pepper Mousse, chilled the mixture in the container for 2 hours, then charged the mixture twice, heard gas release, shook it vigorously 20 times and it would not discharge. On opening the canister, the mousse was not aerated at all. What happened?
You would not have been able to open a fully charged ISI without some effort and a big mess, so I am going to assume that the gas never entered...or entered and released as you were shaking. Disassemble and fully clean your ISI...use the small brush that came with it to clean the nozzle. Check to see that all gaskets are present, particularly the large one that sits inside the top. Reassemble. Fill 3/4 with water (so as not to waste any useful ingredients), charge with one canister. Hold an cup over the nozzle (so you don't spay water everywhere) and pull the release handle to see if gas/liquid is discharged.
My hunch is that you are either missing gasket...or something is caught in the mechanism that didn't allow the valve to seal properly. You don't need much for the latter...black pepper or even raspberry "seeds" will hold the mechanism open enough for gas to escape. Since you say it was your first use, I would guess the former.