Spiders index only static pages:
if you enter dynamic pages, generated by database, etc., they will be refused.
If your website uses these systems, you must create and enter one or more static (html)  pages which contain <TITLE>, Meta Tags and a text which collects all the key words relating to the complete website contents.
Spiders cannot reach the pages contained in directories protected by password. Some Spiders index only websites considered "popular" according to how many connections (links) they have obtained on other websites: therefore it is very important to exchange links with other websites, also as promotional strategy.

The Meta Tag "refresh", used to automatically redirect the surfer to another page, can cause the refusal of the indexing of many Spiders, therefore it is best not to use it.