Price calculation going wrong

Hi, thanks for reading my problem.

I have a website www.yceland.nl
Login (button inloggen) username: test password: test
then click on this link:
http://www.yceland.nl/portal/commerce.asp?CatId=276&ProdId=042FR0KL

the first product is the actual printer every checkbox below is an option for that machine.

If you look closer you see for every product 2 prices.
Adviesverkoopprijs (sellersprice) = fixed price
Uw inkoopprijs (your price) = price calculation

every option has a productpage u can see that if you click on "prijs"

so everything goes well.

When I mark the option and put it in my shopping basket the pricecalculation doesn't work somehow. Real strange is that is works for some articles groups and some not.

The price calculation works this way.
I add a product and sign it up for one of the 20 productgroup i have defined. When a new user signs up I can decide wich discount he gets if he buys with us. I have to put a % on every of the 20 discount types I have. So every price is calculated when the pages is requested.

I will send you the pages on request,

thanks for you r time and help!!
Sander