Making cream puffs doesn't always result in sweet treats for evening tea. If they don't come out right, chances are they deflated or never rose in the first place.
The most likely reason is that the internal structure of the pâte à choux — the type of dough for cream puffs — didn't set right. This will set off a whole slew of issues.
They won't rise if the eggs are not incorporated enough or if the dough is too wet, too dry, too cool when you add the eggs, or too hot when you pipe it.
Dough that's too dry won't have the steam power to lift the cream puffs and won't form a hollow center. If it's too wet, it won't harden in time to hold the puffed shape.
Technique, timing, and patience play a big part in getting cream puffs just right, and the first step to perfect pastries is knowing the mistakes you're making.