You wake up in the morning and notice yourself in the bathroom mirror. Aside from the ruffled hair, you look well enough. Still have a killer face.
The day progresses, you get done the things that need to get done (school, work, chores, whatever). You catch yourself again and notice a red spot on your cheek. There's a zit growing there. It's too early to do anything about it, though. You just have to leave it be and hope it goes away.
In the afternoon/evening time, you have dinner with your friends. It's a good meal, good conversation, all-around good times. When you get home, you brush your teeth and see that your red spot has become a full-blown zit, complete with ugly white head.
When did this happen? When did your cheek start getting red? When did it start growing? When did that white spot happen? Your friends may have noticed the zit, by why did none of them notice it growing?
Writing works the same way (though hopefully it is less gross and ugly). Some projects I could spend three hours at my computer and write two sentences. Nothing has changed in my project. Days and days go by, working a paragraph here and a paragraph there. Then when I want to find a particular passage, I scroll up and realize that there are a lot of pages to sift through.
Somehow, you start with nothing, but add a little bit to it, and it will grow. You may never really see it grow, but if you step back and look at the entirety of your work, you will notice that it has grown.