Preview within Pages showing different page

Hi all – we just set up WordPress, and are converting an existing HTML theme over to WP static pages. We’ve copied all of the files to the theme folder, and set up our landing page as the index.php page and that looks great. When I go into Pages to create other new pages with our custom theme, I put in the code and all looks fine until I hit preview. Instead of displaying the page with the content I just entered/published, it keeps showing the content from my index.php page. Not sure what I’m doing wrong…..everything seems fine until I do a preview, upon which it seems to always default to a preview of the index.php page. Tried creating numerous pages, and even though the content is different the preview always shows the content from the index.php page.