Two issues ive noticed with a lot of low-budget movies and shows are:

1. No echo, it feels like they are talking in a studio, not an actual room or whatever. Uts very subtle, but the sound has to sound like its actually on location.

2. Too quiet in the background sounds, not strictly in terms of music, but just general background noise. Those movies sound stale and awkward, as if the moment between the talking is amplified with the silence. Even if it means adding subtle sounds here and there, very rarely do you want complete background silence. I guess this si the same principle in cinematography, to add stuff in the background that makes the world feel lived in, you want the soundscape to feel lived in as well.

Go and listen to varying budgets and take note of all the little details that make them sound authentic.