The golden rule of trouble shooting to change one variable at a time.
This is hard to do as the number of aircraft changes regularly.
Putting the results in a table may make it easier for your and us to understand.
-10 gain is probably not the best. You probably have this set too high. Check out this and other gain optimisation threads Gain Adjustment
I had to reduce mine to 38db, however, every site is unique. You can use trial and error or the scripts.
If it comes to a choice between two settings, I would try the higher one. It will provide better coverage when it is quiet (like at night or when the weather is bad).
Also, are you sure that the N connector is the 50 Ohm version.
See the pictures in this thread for the subtle difference New, inexpensive 1090MHz antenna for sale - #167 by caius
Look at nearby sites to get a feel for what you could expect.
