Specs - 4400 psi 4.0 GPM machine, 16” 4000 psi surface cleaner

It may seem like a silly question and I’m sure there’s a simple answer, but I’ve always wondered this. With my surface cleaner disc, I’m able to get all of the “surface” level dirt off of concrete, but there’s often deeper (darker) grime and algae that doesn’t come off unless I use a degree nozzle or bleach treatment.

I’m sure the simple answer is that the disc isn’t putting out enough pressure to clean as effectively as the nozzle tip. If I let the disc sit still for a couple seconds, it’ll make a ring that’s lighter than the surrounding surface area. 

Is this correct, and is there no way around this other than one of these solutions? -

a) get a higher pressure machine
b) use bleach, either pre or post treatment
c) just use the 15 degree nozzle

My response to these things are

a) I already am running what I believe to be the highest GPM machine I can run without an external water source. Plus I mostly just do residential driveways so to get something bigger would seem extreme. 
b) I have tried down streaming with many different pieces of equipment and have not found anything that works effectively and efficiently enough to justify the extra time, effort, and cost of using bleach. I may just be too hopeful to think that I should be able to get a driveway clean enough without the use of it. I’ve done driveways before that come squeaky clean without it, but there have been others just like the example image that seem to not come clean without it.
c) It takes too much time this way to even justify pressure washing if not using a surface cleaner. It would easily double or triple my labor time and cut my profits by just as much.

With all that being said, is there something I’m missing? I have tested my equipment and the only thing I could maybe think could be the reason why I’m not getting as much pressure to my surface cleaner is maybe the pressure adjuster on my pump needs to be turned up to get the full 4000 psi to my cleaner. I really don’t know. Again, maybe I’m just being too hopeful that I can get away with not having to use chemical on these extra dirty driveways.

Any help or input is appreciated, TIA

Why can my surface cleaner not clean as much as when I use a regular 25 degree nozzle?