When making choux pastry there are a number of factors to consider when they don't "work":
- ingredient proportions - not measuring ingredients carefully has a huge impact on the final product
- gluten development - not mixing the pastry enough to develop sufficient gluten makes in difficult for the large steam pocket to form
- oven temperature - if the temperature is too low, insufficient steam is formed to make the puffs; if the temperature is too high, the outer surface can bake too quickly and form a crust, which makes it hard for the steam pocket to form
- collapsing - if the product is not cooked long enough or is cooled to quickly, the puffs will collapse