Flat éclairs are a result of either the pastry not expanding enough in the oven or collapsing shortly after being removed from it. ... Delays in serving éclairs result in the moisture from the filling (usually pastry cream but sometimes ganache or whipped cream) seeping into the shell.

How do you prevent eclairs from collapsing?

And ALWAYS remember to prick your baked choux pastry shells (profiteroles or eclairs) with a skewer or a sharp knife and let the moisture inside the cases escape while drying out the shells. This helps prevent collapsed shells, so you can get perfect choux pastry shells.

Why does my choux pastry collapse?

Choux pastry must be thorougly baked. if the sides of the walls are moist, when removed from the oven, steam will condense back into water and the still-wet walls will recoil. This will cause the choux pastry to collapse/ and flattened itself.