Try singing your melody line all by itself, with no drum track, no chord instruments, no instruments at all. How well does your melody hold up a capella?
A capella (pronounced AH kuh-PELL-uh) is a musical term, originally from Italian, that means "sung without accompaniment" or "no instruments, just voices."
Not every rock melody can hold together without accompaniment. The notes might not make sense without the chord line. Rhythmic syncopations might just sound wrong if there's no steady beat for them to play against. And in some songs, the instrumental lines have all the interesting parts, and the vocal lines are little more than punctuation.
I mention all of these caveats to emphasize that it can still be a good song if it fails the a capella test. It doesn't mean you have to rewrite the song from scratch. But if all of your songs fail this test, then you are missing out on the strengths that melody writing can bring to your music.
A song that works a capella is usually one that people can sing along with easily. It's a song that's easy to remember. It sticks in people's heads. It's the kind of song that people sing to themselves while they're out walking and sing with their friends while drinking. Every album should have at a few of these.
How do you write a melody that works a capella? The most direct way is simply to write the vocal line first, and sing it out loud while you're writing it. After the melody is finished and solid, then come up with chords and the rest of the instrumental arrangement. You'll find that the rest of the writing is easy when the melody is strong enough to stand on its own.