You can try a few things - test the gain set at max rather than -10. I found that it gave better results.
You are using a VHF/UHF amplifier, which is definitely boosting signals that you don’t want into the dongle. Depending on what transmissions there are near you, that could easily overload the front end of the dongle and reduce reception since they aren’t very selective. As elpipila says, a satellite amp is better suited since it won’t boost the lower frequencies. Adding a filter might help - either the flightaware one which is specific to 1090MHz, or something like a satellite/UHF diplexer acting as a high pass filter. If I take my filter out I see big reduction in range and messages received. You could try using rtl_power to see what other transmissions could be affecting you. This thread has details on how to do it: ads-b-flight-tracking-f21/filter-not-producing-desired-result-t36540.html
Both satellite amps and diplexers are dirt cheap on ebay, so it’s worth having a look there.
The flightaware antenna is pretty good, so you should be seeing better results than you are, but it’s important to give the antenna an unrestricted view of the sky - if it is inside, try putting it in the loft, but better is to have it mounted outside as high as possible. Surrounding buildings or trees can obstruct the signal and reduce the range considerably.