You can improve your whip antenna:
How to improve performance of whip antenna of DVB-T
Whip antennas are very bad though, even a simple selfmade antenna is much better:
QUICK SPIDER - No Soldering, No Connector
Placing the antenna outside above the roof line is a big step up no matter the antenna you are using.
The Flightaware antenna is again quite a bit better than the quick spider but you can reach very good range with the quick spider mounted on top of the roof.
You might also look at how terrain around you is limiting your view of the sky:
What is the Maximum Range I can Get?
For experimentation these graphs come in useful to compare for example two antenna positions:
GitHub - wiedehopf/graphs1090: Graphs for readsb / dump1090-fa / dump1090 (based on dump1090-tools by mutability)