Thanks for the replies. I'd rather not upload the .qvw since the data is my clients and I'm under quite strict NDA.
However, I think I got it working by applying AGGR function to Amount and Qty as well:
Amount: AGGR(Sum(Amount), Product, Supplier)
Qty: AGGR(Sum(Qty), Product, Supplier)
Hit Ratio: AGGR(sum(Missed)/sum(Offered), Product)
Solution provided by Sunny might work as well but I haven't tried it yet.