First you need to check the front of the blade, right up to inside the stock. If there is a crack there (which has happened to me and a few others in the squad) then the rudder has taken on water and has become bendy. The only solution is to replace it.
However, hopefuly this is not the case, and this is how to set up some race winning rudders:
1. Put some packing around the bolts in between the rudders and the stocks (a fairy liquid bottle cut into a disk with a hole in the middle for the bolt is what I used) and tighten the bolts. Tight as poss, but still so the blade can be put up and down.
2. Loosen the bolt on the top of the rudder stock with the rudder down and on the boat (need to lift the back of the boat to do this). Then put a screwdriver in the hole behind the bolt and push the plate as far forward as you can and then tighten the bolt again. This pushes the tip of the blade forward reducing weatherhelm.
3. Still with the rudders down, measure how parallel the rudders are. I sailed with the back edge 8 to 10mm further out than the front and it worked for me. You adjust this by changing the lenght of the tiller bar.