A lot of the products are grouped products, with grouped products associated. Don't know how this is possible, if he worked via database.
In the website version the products are showing up in the frontend, but in my copy of the site, after the reindex process and after cleaning the cache, the products disappeared.
Another thing, i added a new attribute to my products, but this don't seems to be the problem.
How can i always show grouped product in the frontend, with or without associated products?