OBS can only listen to the first two channels of any sound card. It's just the way it is right now.
I have this issue with my Midas MR18, that sends 18 channel through UBS to the computer, but I've gotten around that by routing my Main Out to USB 1-2, in order to get my main mix into OBS. It works. Maybe you can do that in the Presonus too?
I've gotten used to this limitation now. I'm streaming channel 1-2 (that's my main mix from the mixer) to OBS at the same time that I'm recording up to 18 channels into Logic Pro, on the same computer. They can both have the MR18 as an input source. OBS will only hear channel 1-2 but Logic Pro can record all 18 channels.
There is also a new free virtual soundcard called Groundcontrol (for OSX only), that will let you patch any audio channel from any soundcard into a virtual two channel sound card that you then can get into OBS. That's a workaround if your main mix is locked to channel 15-16 or something. Don't have any experience with Windows unfortunately, but some kind of virtual sound card (for routing) must exist there too.