The front right burner on my cooktop will not light even though the ignitor is sparking and gas is flowing (I can light with a fireplace lighter).

I'm not aware of any boil overs on this burner and have thoroughly cleaned the burner base, head assembly, and electrode (including an emery board on the metal tip).

The other 4 burners light without issue.

Does anybody have suggestions as to what the problem might be?

Thank you.