I am trying to hang a farily heavy window pole. However, I always have problems fixing anything around my windows as there appears to be something really hard behind the initial plaster layer (which is about 15mm thick) that I cannot drill into, even using hammer mode. This is not just where the lintel would be. Obviously, 15mm of plaster is not a very sound medium for such a task and it isn't long before the plastic wall plugs pull out. I have tried to use thicker scews but am limited by the diameter of the holes in pole brackets. I also thought about using a strong glue around the plugs but don't really want to pull the house down when I come to change the pole or re-decorate in the future. Any ideas ?

They may have installed a steel lintel over the window. If so, use a titanium nitride bit, still and tap the hole.

They may have installed a steel lintel over the window. If so, use a titanium nitride bit, still and tap the hole.

What do you mean by "still and tap" ?

Sorry, big fingers on a Kindle Fire "Drill and Tap"-maybe I should read more!!