I have the same issue, mine worked fine for 3+years then just stopped working. Replaced the gas with a genuine gas, it appears to carbonate, if you hold it down long enough it it hits the bypass valve etc but no gas at the drink.
When looking at some YouTube videos for the typical fix [https://youtu.be/st4d6drjVeE] . I didn't get the backpressure of gas out the back, only a slight hissing noise.
I tried in on a mates Sodastream that was working correctly and there's had a fairly large rush of air (like in the video).
I guess I have a blockage which is too stuck for the gas pressure to dislodge. I'm yet to disassemble mine to look at the issue but thought I would post here first.