So, if the panna cotta hasn't set there are two likely culprits: 1) not enough gelatine for the liquid 2) not enough time in the refrigerator The ratio of gelatine to liquid is important in order to ensure a dessert sets. For this recipe, if you use too little gelatine powder, you risk this not being adequate for setting the dessert. Similarly, if you use too much liquid (adding extra yogurt or cream), this may cause the gelatine to be insufficient for fully gelling the final product. Another common issue is not giving the dessert ample time to set up in the fridge. This is why I recommend making this the night before. If your panna cottas are still runny after an overnight in the fridge, unfortunately the gelatine has not worked properly.