Llap, had similar issue with one of my mirorrs until I replaced both with Lucas type.
There is a tapered angled interface between miror itself and stem which needs to be screwed fully home tightly. Unfortunately there are no flats on this part to aid tightening, so try twisting mirror itself clockwise until its tight onto the stem. A soft jaw Mole wrench may also work direct onto interface if careful not to damage chrome.
You can still move mirror in/out but may then need to adjust the mount itself onto bodywork, as described by DocNik, to gain desired rear view. Always adjust up/down with clockwise movement of mirror itself (it will continue to turn stiffly through 360° after fully tightened) to avoid it loosening again.
