I'm not familiar with the specs. of your receiver, however ......
The APT bandwidth is 34 kHz.
I usually use 38 kHz for SDRSharp to allow for some doppler shift.
If you can tell that you have a strong signal, then the bandwidth may be your problem.
But, it looks to me like you have BOTH problems. A weak signal AND a bandwidth that is too narrow.
Rich
NOAA-18 APT reception and settings.
Edit: Here's what the processed image looks like.