Weird, I'm the opposite. It takes a bit of effort for me to come up with a good melody but once I have that I'm able to have the basis for an entire song within a couple days.Again I sit with my guitar to complete my first composition and come up with too many stuff and fail to arrange. I didn't believe when musicians said that coming up with melodies was the easy part, turns out they were right.
Hundreds of material and 0 songs. Drives me nuts.