Is the lamp fixed/installed correctly? – Are the seals tight and are screws and glass tightened sufficiently?
– If it is a garden/bed lamp then rising soil moisture in the lamp tube can possibly cause condensation. This can be remedied by filling a sealant in the tube. Polyurethane foam like PU597 is recommended! – Drainage holes must be kept free from dirt. REMEMBER TO DRY ALL THE INDIVIDUAL PARTS BEFORE THE LAMP IS PUT BACK TOGETHER!