zoom in/out

i'm doing somehting on openGL.
i try to zoom in and out my object(let say a cube).
when i press the "+" key ,the object is suppose to scale up 0.1
then the zoom up object will be display.
when i press the "-" key ,the object is suppose to scale down by -0.1.
However, when i press the "+" key ,then the "-" key,i will need to key in "-" twice to see the scale down effect.this applies also when i key in "-" then "+".
why is it so?
my code:
void CDhView::OnKeyDown(UINT nChar, UINT nRepCnt, UINT nFlags)
switch(nChar) {
      case VK_ADD:
            if (scale <= 5.0f)
             scale+=0.1f;
            cameramode = PLUS;      
            break;
       case VK_SUBTRACT:
            if (scale > 0.0f)
               scale-=0.1f;
               cameramode = SUBTRACT;
               break;
            }
void CDhView::OnKeyUp(UINT nChar, UINT nRepCnt, UINT nFlags)
{
switch(nChar) {
case VK_ADD :
      cameramode = STOP;                        
      break;
case VK_SUBTRACT:
      cameramode = STOP;
      break;
}
}                        
void CDhView::OnDraw(CDC* pDC)
      //Zooming
      if (cameramode == PLUS)
            glScalef(scale,scale,scale);

      if (cameramode == SUBTRACT )
            glScalef(scale,scale,scale);

thanks.