So define a sad vs a happy plant more specifically.
What might be the issue there?

You need to provide similar conditions and acclimating time for the plant comparison also.

Sad plants often are stunted and take a long time to recover. Sad plants often do not have good growing conditions to begin with for that plant, some plants, say Hydrilla or Milfoil are very strong competitiors.

In a tank with very few plants, I'll assume such a tank si non CO2, generally a fish tank with some plants added?

In such cases the sad plant gets beaten out for nutrients.
Thus the sad plant is beaten out for nutrients.
Then it's only a substrate for algaer growth like a rock or the glass etc.

The healthy plant is able to out pace formation of the algae, even Anubias are able to do this.

Regards,
Tom Barr