Hi Marit, Usually the article-class should have page-numbering on all pages, so probably something has been changed from the standard, which might make it difficult to know exactly what is wrong. However, the most probably is that Document->Settings...->Page-Layout->Headings-Style is somthing other than Standard If it is set to Standard try fancy.
If this does not fix it, you have to send us a copy of your document, you can remove all text, but leave all markup and "code" (we call it ERT on the list :) ) Ingar