The yaw string is a useful device, but when learning to fly, it's far better to look out at the horizon and use that as a reference for rudder application. It gets the pilot's eyes outside and the real horizon is very easy to use as a reference. May I suggest that the reason you are not using the correct amount of rudder is because you haven't learnt to look out properly (or rolling on a heading hasn't yet been demonstrated to you) and not because of a change of type. Flying a different glider does set some people back, but generally not for long. When you have the correct proportion of aileron to rudder, the glider will roll on its longitudinal axis and there will be very little yaw. Next time, when you turn (after your lookout) look to the horizon and apply the controls. Too little rudder and the nose will yaw away from the turn. Too much and you'll get yaw into the turn.
As for the effectiveness of the rudder, well anything you are likely to fly in the near future will have enough rudder. I say that after having flown a K13, Twin Astir, Acro, Janus, ASH25, K21, T21, T31, Caproni Calif etc. Our German friends have pretty good designers and test pilots (even Grob - I'm not a fan. But flying one is is like spending a night with a Kangarillapig, you'd do it but wouldn't tell your mates afterwards) and make sure that their products are properly controllable. Otherwise it wouldn't be sold.
PM