Can you check what weight you are using, there should be a number at the bottom of your spring, where the foot is.
You should be using at least 80kgs or better would be 90Kg ones.
Also just noticed you mentioned you have middle sized one, the stilts come in either kids versions or Adult versions - If you have the kids type you really shouldn't be using them as ur too big
but if you have the adult ones & the correct weight they should be ok for you - remember height comes with technique and that can only be achieved with practice!
below are pics showing the two models: