kuttingchai
why is "one of the key ingredients" not the subject?
consider example:
One of the apples in the basket has fallen down.
Here "One of the apples" is subject which is singular and "has fallen" is correct
These are not the same sentences. In fact, our instructor discussed this in great detail in S-V class. So, here is the thing:
Statement: Clear communication is one of the key ingredients that are often lacking in romantic relationships
This has two clauses:
1st clause: Clear communication is one of the key ingredients
2nd clause: that are often lacking in romantic relationships
Statement: One of the apples in the basket has fallen down.
This has only one clause.
In other words, if I had to change this sentence as:
One of the apples that are in the basket has fallen down.
Now, there are two clauses:
1st clause: One of the apples has fallen down
2nd clause: that are in the basket
Again, meaning is the same: There are many apples that "are" in the basket; one of them has fallen down:).