This comes up all the time...and just recently it seems to come up a lot with people using SFX-1
Yeah, if what you're doing isn't removing the flaws then you need to either get more aggressive or spend a *lot* more time doing what you're already doing. Within reason I'd generally recommend the first solution.
More aggressive (when you're already using speed 6) means a different product and/or different pads. The smaller 4" pads perform more aggressively than the larger ones and also allow you to put more pressure on the PC (which some might not consider advisable). FWIW, this is about the only way I can do serious correction with the PC. I almost *never* try to remove marring with the PC and larger pads, it's just too mild an approach for most of my vehicles.
Products like Hi-Temp EC and HC, which seem to be the flavors of the month right now (and for good reason IMO!), used with a 4" pad, will do about as much work as you *can* do without a rotary. But I'd try using the SFX-1 with a 4" pad before you try different products, it might work fine.