Open Question: Getting rid of pixels in photoshop?

I did a project in photoshop a few weeks ago ...it was a poster, and had many different layers, etc.. no matter how big or small i would make the next layer, the image would always restore itself so that there would be little to no pixels. I'm doing another poster, but this time all of my images are becoming very pixelized no matter how big or small i make the image. Is there some sort of option that should be checked off to prevent this from happening? How can I make my images not pixelized anymore? My other poster turned out just fine .. :(