Resize problem when maximizing a MDI

Dear Reader,

Can anyone give me a hint where the root of next problems lays ;
I have a MDI application created with Visual C++ 6.0. This consists of 4 splitter windows. One is a OpenGl rendered view. When maximizing the window containing the 4 splits I do have a unrendered area on the right side. When I ones force a resize by moving the splitter the above problem does not occure.
This is the standard resize function I use ;
//-------------------------------------------void CDads3DView::OnSize(UINT nType, int cx, int cy)
CView::OnSize(nType, cx, cy);
miv_WindowXSize = cx;
miv_WindowYSize = cy;

if (cy > 0 && cx > 0)
	glViewport(0, 0, cx, cy);
	m_oldRect.right  = cx;
	m_oldRect.bottom = cy;

	   gluPerspective(45.0f, (GLfloat)cx/(GLfloat)cy, 500.0f, 6000.0f);

Any help is welcome !

Best regards,