Why are my sponge cakes not rising?
Seeing your recipe would be helpful here. Older sponge cake recipes rely only on beaten eggs for leavening. Newer recipes use chemical leaveners as well.

Common reasons for sponge cakes not rising (or not staying risen) are:


Not beating the eggs long enough
Even with the addition of chemical leavening, getting enough air into the eggs in advance is important to the characteristic structure of a sponge cake. The eggs should be very pail and fluffy. Some recipes explicitly call for beating the eggs for at least 15 minutes.
Old baking powder
Baking powder can lose its effectiveness after a long time. This is easy enough to test. Just mix some baking powder with warm water and if it bubbles actively it's good.
Old oven
My oven recently started struggling to keep temperature. I had to recalibrate many of my recipes until I fixed it. Some things like sponge cake are very sensitive to this because they rely on the initial blast of heat to produce the steam that blows up the bubbles in the batter before the eggs set.
This is also easy to test with a cheap oven thermometer.
Opening the oven early
If the oven is opened when the cake is just starting to puff up then that high heat will be lost and the steam will escape from the unset eggs and the cake will fall, never to rise again.
I realize this would be a change in your behavior which may be unlikely but it is a common reason for sponge cakes to fail.