I have a few of these in my basement setup.
Ensure that the mcx connector is firmly plug into the mcx/sma adapter.
I (and others) found that these antennas are finicky and a slight movement alters their effectiveness.
Do you have a good groundplane base? I know someone did some testing, however, I can’t find the thread.
This thread has info on optimising gain
ads-b-flight-tracking-f21/gain-adjustment-t37172.html
The script will give you an idea if you are way off.