The original question I was trying to answer was why someone is missing planes on the ground. You need the gain low + you need the receiver location to be set.
I forgot to mention one more thing. Old receivers (8+ years ago) had a huge donut effect. The minimum detection range was around 15km and their max range was around 200km. And they cost in about $600.
Today the receivers don’t have a noticeable donut effect (3km to 400km decodable range) and they cost $20.
I would just set the gain as high as possible and not worry to much about missing the very few close by messages unless you really need them.
Here is a lot of discussion about gain and number of messages detected.