More holding leg and less nudging leg? Ensure you are not turning/weighting your body/seat bones too much? Check on your hand/shoulder position and rein tautness...
Try ignoring the half pass to start with.
Ride a straight diagonal line between, for example K and C, keep repeating this so your horse is anticipating going in that direction.
Eventually, while doing exactly the same thing, push her quarters over for a few strides then straighten her back onto the diagonal line.
You can then increase the amount of strides you are pushing her quarters in for.
Think of pushing over with the outside seatbone rather than the leg, the outside leg should be behind the girth but completly relaxed. If the outer leg is too rigid or too stuck on then that kills the forward movement ( as weezy said) At the same time both the hips have to be completly level with one another and straight.To keep them forwards sit a little bit on the back parts of both seatbones.Keep tall with the stomach pushed outwards without allowing the spine to curve. Its seatbone control mostly but does not work with a slumped position or with any friction in the back. Sorry does that make any sense at all>?!
When riding a half pass no matter what pace it is, think of riding travers on the diagonal.
The problem you probably have is that you have to much outside leg on and a slightly restricting hand. Try and ride a few strides and then go straight and then a few strides again. This encourages the horse to always think forwards, which the half pass must be. when u have him happily changing through the half pass (i.e few strides, straight etc) then play with the tempo when you have a half pass without to much angle, collect, then into medium, then collect again, he'l have to think forwards because you are changing tempo.
If he ducks behind the bit, because the angle is to great, think about lifting his tummy upardswith your leg, and then a check up and forwards with your hand! Do you have mirrors? They always help because you can see if you are tilting etc.