Why is this page not found by Google?

Of my website http://www.schullercompany.nl made up in php one of the pages http://www.schullercompany.nl/?p=Financiële-administratie is not indexed by Google. All aother pages of this site can be found easily by Google. It is there already for more than 6 months.

What do I do wrong with this?

Not to sure with out looking for my self but lets go through the obvious first (Just in case) ~
Does the page have any of the following ~

1. Included in your robots.txt file?
2. Does it have a meta exclusion in the header?
3. Is it similar to other pages?
4. Does it use a JS redirect?
5. Is it linked to from any pages? - internal or external
6. Do you use a 302?
7. Have you recently implanted a 301 recently?
8. Does it generate anything more than a 200 on a regular basis?
9. Does it have lots of HTML errors or does it validate? long shot but if it does generate lots of HTML errors in the code the bot might be chocking on the code and giving up.
10. Are you using blackhat - hiding via css, keyword stuffing etc.. on that page.
11. Does it have any content?
12. Is it accessed from a form?
13. Is it password protected as in login?

If none of those work you could try adding an extra page for example rather than
http://www.schullercompany.nl/?p=Financiële-administratie

call it in via
http://www.schullercompany.nl/subpage.php?p=Financiële-administratie

If none of those don't work I'm not sure.

Maybe some recording in robots.txt or simple google do not handle such URLS.
I think that there are many documentation to find.