You might be having trouble if you're not working with vocals. If it's just guitars you might be trying to fill in the melodic gaps too much with busy rhythms etc. Some of the best rock songs ever are two, three chords tops but the vocal just makes it. Imagine trying to sing over what you play and you'll find it's much easier to come up with melodies when the guitar is simpler.
Perfect example of this is AC/DC. Bad Co. is another good example. Also Kiss.
A lot of times when you figure out a classic song you'll be surprised that the actual part doesn't have notes/chords in it that you were positive existed. It's because between the bass, drummer and vocals a lot can be suggested that actually isn't there.