Unable to drill into window lintel - what have I got wrong?
Plate steel has been used to support lintels when the span is over some distance.  That detail is obsolete now un the US but I'm not sure when they stopped doing it.  Now-days its plenty of re-bar and in lintels it is often caged.   Rebar should not be closer that two inches (5cm) to the edge of the concrete.  Maybe there is an engineered steel bracket embedded in the lintel.  Either way, you are likely running into steel.

They sell rebar eater bits, assuming that is what you hit,  they can cut through steel but if you hit the bar dead on then even a rebar eater is going to have some work do grind through it.  Only use them on rotary, percussion destroys the carbide teeth on them.  Maybe you can move your hole a little?  Maybe use a smaller inexpensive steel twist drill you don't mind throwing away in the same hole and observe chips that come out, looks like steel?