MDI Child at toolstripcontainer

Question

Hi probably a very small issue - but I have no idea... I added MDIChild Forms to a toolstripcontainer ala:

f.MdiParent = this;
this.toolStripContainer2.ContentPanel.Controls.Add(f);
f.Show();

This works quiet fine, but all the sudden I cannot activate this childs, i.e. Clicking to one of these windows will not keep it active. The window handle (top bar) will not stay blue - it remains grey and the window is not active. And so key events will not handled.

How to do?

Best regards, T.