Couple of things here.
1. Yes, the antenna needs, ideally, to be outside, especially if yours is a modern house with aluminium clad foam insulation between the walls - the best possible RF shield! I have mine by a south facing window and I see far more stuff to the south than I do elsewhere.
2. The supplied antenna is actually too long for ADS-B on 1090MHz, It is resonant at about 800MHz and like any antenna its performance will deteriorate markedly either side of resonance. I cut mine down to be more or less the correct length for resonance on 1090MHz. It made quite a big difference! Whip length is 7cm. You'll get a further improvement by mounting it on a ground plane, e.g. a metal tin lid.
Unfortunately there are no "magic bullets" with antennas: they need to be resonant and they need to be in the clear to work properly.