It may have something to do with Screen->LayerMap() or Screen->LayerScreen()
Did you try putting in the values manually?
Also what solidity value are you using?

This is the function from std.zh
Code:
//A shorthand way to set a combo's solidity on the current layer.
//Layer 0 is the screen itself.
void SetLayerComboS(int layer, int pos, int solidity) {
  if (layer == 0)
    Screen->ComboS[pos] = solidity;
  else
    Game->SetComboSolid(Screen->LayerMap(layer), Screen->LayerScreen(layer), pos, solidity);
}