I've been very happy with my Nacra pivamatic rudder pull down system on both the Nacra's I have owned. I can always bottom out the rudder and it stays put and only comes up when striking something solid (a large jellyfish can sometimes release them). The only mod's I've made is to add a lawnmower style pull handle to the end of each line. Can you describe exactly what is happening? Maybe we can figure out what the difference is.

I've seen some folks put a 2:1 purchase on their pull downs but it takes a lot of line. Catsailor sells a one line kit that will pull both rudders down with one pull (I've never seen this in action but I believe you loose the function of the pivamatic).

The cleat can be very sensative to the type of cord you use - the cheap stuff works best in my opinion - I use 1/4" rope from Lowe's for the rudder pull downs but it's the only place on my boat I use this line. Perhaps your castings are bolted too tightly to the rudders? The rudders should swing freely in the castings - I use large plastic washers (also available on catsailor) to shim up and provide a bearing surface between the casting and the rudder instead of cranking the casting tighter to the rudder.