That whip looks a tad long from here 
You have graphs1090 installed?
Or tar1090 and have a screenshot of /?pTracks?
Both will help checking for range improvements.
So you could bend the top 3 or 4 mm of whip in on itself so it touches and check if that improves reception.
G7RGQ Antenna 1090MHz
This is a nice homebuilt as well .
Or this one:
QUICK SPIDER - No Soldering, No Connector - #111 by abcd567