Hi Eve,

In this case, 'bon', acts as an adverb, not an adjective and doesn't agree with the subject.

You could say for instance -

Ces fleurs ont une bonne odeur. 

but it is more French to say -

Elle sentent bon ( ou mauvais) = They smell good ( or bad) 

Hope this helps!

Hi Jean, 

This is already mentioned in the lesson -

Note that, in the specific case of sentir, even though bon and mauvais are adjectives, these are fixed expressions, so bon and mauvais NEVER change (no agreement).