How long has the beer been in the keg? Do you carbonate at serving temp?
I've found that with higher serving pressures the beer will initially seem carbonated but has poor head forming and retention. Given time (maybe a week or so) the head seems to stabilize.
What I think might be happening is that the higher pressure is forcing the gas into suspension, which is why the beer seems carbonated, but that it's not fully dissolved into the liquid when I initially tap the keg. The pour is good, the carbonation seems to be there, but I get a thin fizzy head that dissipates quickly. A week or so later, it's a different story.
So, maybe you just need to be patient?
The beer has been in the keg now about a week at 30psi at 40F.
The root beer is clear as well so there is is nothing suspended and yes, it is a full 10% ABV.
Like I mentioned, the co2 appears to be fully absorbed as there are a ton of bubbles rising from the bottom of the pour from start to finish and it tastes well carbonated.
The tiny thin head that is forming is not dissipating quickly and there is nice lacing as you drink it.
I guess I'll see how it develops over another week or so but I'm thinking it has something to do with the root beer extract oils and ingredients that might be limiting the head production.