You need to try Cat Attract (PetsMart carries it & it's available online). Cat Attract comes in a shaker you can add to your regular litter or you can buy the actual Cat Attract Litter. There is something in the litter that pulls the cats into using the litter box. They say the kitten formula works especially well when you're trying to litter train a new kitten. My cat is up in age so I use the adult formula litter. I was able to get one of the large sized Omega Litter Boxes, which you turn sideways and the good litter goes through a grate while the clumps go into a tray you take out and dump. I found that even with the pricier Cat Attract litter, it actually lasts longer between dumping the entire litter-box and washing it down before setting it back up again so in the long run, it actually saves money and the cats go back to using the litter-box.
Is your current litter boxes in quiet areas of the house? With that many cats, you need at least 2 if not 3 separate litter boxes, especially for the one that won't go to the litter box. Make sure you are diligent about cleaning them all out multiple times a day or they will abandon the box too.
My cat (Kitt's about 8-9 years old now) stopped using his litter box for a while too. That's when I learned about Cat Attract. It got him back in the box. I bought it at the best time too because we were remodeling our bathroom where his litter box was for a long time. We had to replace the flooring in the bathroom anyhow because of some damage from a leak from the tub but once we got the sub-flooring up, we learned that he had been spraying (peeing) outside his box. He has only had 1 accident since we moved the litter box to the laundry room and that has been about 6-8 months ago.
Give it a try and also try to add another litter box to see if that helps. It could be that the cats may be fighting over the litter boxes. The largest and usually the oldest cats take priority in their little cat pack so the youngest and newest of the brood will have to fight for a higher rank in the pecking order. Usually adding an additional litter box will help matters tremendously.
Hope this helps.