How to repair <!–nextpage–> problem

When attempting to use this tag to split long posts and pages I encounter the following problems. Note I have placed the correct code in both my themes page.php and single.php files:

In one page it will split the page but the second page does not carry the original font or formatting (ie:blockquote)

In the second instance the first page is split correctly, but when you click on the second page what you get is not the second half of the text but the entire article with no link back to the first page.

Does anyone know what may cause these problems. It seems using this tag is the only way to split long posts and pages.