I keep trying to mount my RPi and Prostick directly to my FA antenna on my tower and every time I get 1/2 of the messages even after adjusting the gain.
Ya know what would be interesting, do some wideband scans with something like rtl-power to see what kind of signal environment you're in. You might (probably) have some high power sources nearby, cell towers, digital TV, business band stuff, that is overloading the front end of the Prostick.
The prostick front end has a (wideband) LNA, then the normal SDR. The Prostick Plus adds a very narrow ADS-B filter between the LNA and the normal SDR.
Because of the wideband design of the SDR, strong out of band (non ADS-B) signals can turn everything into garbage.
I know in my environment, I get better results with the FLightAware filter between the antenna and my Prostick Plus SDR because of local transmitters, which include businesses, FM, and cell towers.
If you find culprits on a few select frequencies, you can look at what kind of filter to use to knock them out. I'd probably order one of the FlightAware filters ($20 from Amazon if you're in the U.S.)
Experiment and let us know what you learn.