At the risk of respectfully contradicting some people here, the brush is somewhat at play in this case. I also have an HP-CH+ that used to be my go to brush many years ago. I rarely use it now, using either my Microns or my Eclipse. The reason is that for some reason or another the HP-C throws a lot more paint, and takes more pressure to make it "go". It is a fantastic brush for all around use, and is great at fine lines on porus surfaces, but the volume of air needed to get the paint moving on hard surfaces ALWAYS caused me problems.
I downgraded to my eclipse for any fine work on hard surfaces until years later I bought a micron. I don't think it's a matter of atomization, as the atomization of paint off the HP-C was always smooth as silk and let me do some of the best subtle color control work of my life, but absolutely, to get the paint flowing well, I had to push up the pressure to about 18 to 22 psi in order to get air volume, which is just too much for any close detail work on hard surfaces.
Note that I am talking about volume of air, and not pressure exactly, but volume. They are related, but not quite the same thing. If you have a good tank compressor that can move a lot of air at low pressure, then this isn't a problem. But if you have a smaller compressor that has to up the pressure to move the same amount of air in the same time, this can be tricky.
I think that the inner cavity behind the nozzle cap is greater in the HP-C series, or at least that is my impression. It is fantastic for atomizaton, but trickier for control.
Now, you can mitigate it to a large degree by really over-reducing the paint, but that comes with it's own risk of spidering. With time, and a lot of practice, you certainly can tame the unruly beast and get fine lines on hard surfaces as a few have said above, but the learning curve for it on an HP-C is a LOT harder than with an eclipse or micron.
In my opinion being the proud owner of a lot of different Iwata guns, the HP-C are really optimized for medium density paints on traditional fine arts or illustration surfaces. You can get them to perform reasonably well on hard surfaces, but it will take a lot of practice with trigger control and experiments with paint reductions and pressures to get the detail you want. Honestly, I can say I found that easier with my eclipse.
The micron is great for fine lines on just about any surface, but even then, you need really good trigger control to take advantage of that, and you need to use very reduced paints, which means a lot of passes for coverage.
With the right surface, paint, and pressure the HP-C brushes can perform like a micron, but they are much more finicky about the conditions they need to get there.
It will come with practice. It just takes some patience.