This topic fascinates me. I have no trouble writing songs. It may start with 2-3 chords, or a something I over hear while someone is on a cell phone or at the market "I told her'baby you took the wrong exit'" It may start with a morph of something I hear on the radio, or even a chord I hear in a Pink Floyd song or the intro to a Stones song.
The problem I'm having however is how to notate this stuff. I can write down the lyrics....I can draw little pictures (how professional!) of what the chords look like on my fretboard. Yet even with that, if I don't play it about 50 times at first, It will shift and I have a hard time stabilizing it...if that makes sense...getting it to gel. Am I the only one with this problem? I write it all down. Come back the next day and I realize the phrasing is gone, not sure what I accentuated before. Not to mention my timing can be wonky