According to RIAA, the last time any Styx album release (outside of GH) was certified in a platinum category was in 1984. You would think that 27 years of sales, one release (GI, P08, Cornerstone, PT) would have achieved enough sales to go to 4x platinum? Heck even Greatest Hits has stalled at 2x platinum (last certification 2005). Do the math - you mean that Greatest Hits can't sell 15,000 copies a year? Or that Grand Illusion hasn't averaged selling 4,000 copies a year since 1984? I find that hard to believe.
Maybe RIAA just doesn't certify older albums anymore?