problem with group priceing (per item)

[modchipfitters.co.uk]

can anyone help i have installed group_pricing_per_item_1.2.1a on to a zen 1.3.7 installation
the problem i have is all discounts are working when the customer reaches the checkout but group d a group c wont display the correct prices when browsing products yet groups a & b work fine

for example if a customer is assigned to group a or b and the default price is 9.99 and group a price is 8.99 then the customer for group a will see 8.99 both on the product listing and in the shopping cart and in the checkout ok all works great so far
but if i assign the same customer to group c or d they it wont display the correct price on the product listing. to clarify if groups c's price for the 9.99 item is 7.99 they will see 9.99 when viewing the product listing but once in the cart or checkout it shows the correct amount of 7.99

are you all totally confused now??? i know i am!! lol
anyone got any idea's????

[modchipfitters.co.uk]

still not managed to figure out the problem can anyone please help as i have lots of customers signing up for our trade site but at present i can only offer them 2 pricing levels till this problem is sorted oout