Sounds like you emultating in Windows, if so WinUAE? To get to 16Mb and more use the Z3 slider not the Z2 for fast ram.
Sorry did not see this was for the GS, apologies!
But the same reason applies, the memory settings are for basic fast ram / Z2 so only go up 8 Mb each or Chip and fast and only 2Mb chip if more than 2Mb fast
