Flat (undercarbed) beer pushed with beer gas through a stout faucet will not produce any head. As it becomes more carbonated, it begins to produce a head with the pour. At some point in the process, it will go from no head, to little head, to a little more head, to perfect cascading head, and if you allow it to carbonate further, all head, which is about where you are now. This is because the sweet spot for nitro pours is a very small window between too little and too much carbonation.
Since you are a beginner WRT beer gas, the easiest and safest method would be to do as masonjax recommended by putting it on the mix gas from the start and patiently waiting 2-3 weeks for it to carbonate. Once you have more experience and get a better feel for how the gas works, you can then use CO2 to speed the process along. About 24 hours on CO2 @ 25 PSI, then moving to the mix gas @ 30 PSI should have you getting nice cascading pours within about 1 week. However, right now you need to work on decarbing your beer and then bringing it back up to the sweet spot.