I do that all the time, and it shows by what I have uploaded, I have a lot of "teaser" or "Sample" clips, that rarely, to never, get updated to full songs.
Like said above, sometimes all it takes is time off from that particular song, or a sudden jolt of inspiration.
One thing that you should also take into consideration is the composition of that first bit of the song, as I found out with my song "Journee", you can sort of trap yourself with how you compose what would usually transition into the rest of the song.
I personally find if you throw a climax into the early parts of a song, it is VERY difficult to build upon it, as the song already feels like it should be dying down, which is why the song mentioned above has somewhat of an identity crisis and swings from mood to mood, is because I couldn't find a way to build on the intro.
All in all, give it time, some people take hours to finish a song, others take years. When you start writing a song you're serious about, don't expect it to be done right away, let it sit for a couple days then go back to it and try again.