I've noticed a similar thing, when I'm composing for keyboard. If I write the first two lines with nothing started on the next, when I come back the new writing sounds completely different. That's ok if you're starting with a new section but not so easy when it's supposed to be the same one. A good solution is to sketch out the melody for the next part and leave it incomplete, or at least know what you want to do next before getting up from the keyboard. Ymmv.