What's at the bottom of the render? Are there a few courses of bricks above the ground before the render starts? I've used double 200mm x 75mm timbers bolted together as lintels on timber framing and you really don't want to drill through those. They could be picking up the load from the teh roof, you really need to be at least 200mm from the window/door reveal to the edge of the hole before you start drilling (professional wood butcher)