Try it with a different instrument. You can just pick an synth or sample and drop it on top of the current one in Song Editor.
Try speeding up/slowing down the tempo. Hover over the tempo indicator and use your mouse wheel. I'm finding that 160 seems to work for me most of the time. Kind of depends you. Like everything Music-y, it's all about what YOU like.
Add percussion. Even if it's just BOOM-crash-BOOM-crash. Makes an amazing difference. Use the BB editor OR NOT. I'm a NOT person, myself, but that's just me. There are songs in the Sharing Section that are made ENTIRELY in the BB editor. And there are songs that don't use it at all.
Try using some chords. Several ways to do that: if you're using a synth, one of the standard "effects" is to play each note as a chord. Experiment with what sounds best to you. Also, in the Piano Roll, you can just "turn on" your choice of chord. Place a note and it places a chord instead. Or do what I do, use ctrl-c to copy everything in the Piano Roll, then shift-drag a note up or down (while it's playing) until you like it. Hint: LEAVE ALL THE NOTES SELECTED (blue) UNTIL YOU'RE SURE - so you can delete your added chord all at once. Else you'd have to delete each individeual added note... Even safer, copy your blocks in Song Editor further over so you can always just delete it entirely if you mess it up.
Play your melody on a loop, and change the notes (drag them up/down/around) you don't like while listening to it. I'm often surprised what "works" that I wasn't expecting.
Try to copy someone else's music that you like. Just what you like about it, not note-for-note. See what happens. Even if what you end up with sounds like an obvious cover, so what?
And lastly, something I read but haven't tried yet, just write your melody all in middle-C, then commence dragging the notes up/down until you like it.
Making a melody takes me a while - I'm not musician enough to know ahead of time what will and won't work. I often get sick of listening to it looping while I'm working on it. Which is a good excuse to stop, save, and do something else for awhile.
I never thought I could write an original piece of music... Working on my fifth song now.
I call it "The Fifth One".
And *I* really like all of them. A lot. Well, the First One isn't ENTIRELY original... but it's pretty different from the real original ("If I had a Heart" - Fever Ray).