reason that comes to mind immediately is that the temperature
inside your oven differs from the temperature indicated
on the oven dial. All ovens need recalibrating occasionally.
If the oven temperature is too low, the structure of
the cake may not set sufficiently, even though it may
have been in the oven for the recommended amount of
time. Thus, it collapses once it's removed from the
oven. You can check your oven's accuracy by buying an
inexpensive oven thermometer, found at hardware and
cooking stores. Sometimes it's surprising to find a
considerable temperature discrepancy between the dial
and the oven.
Also check for doneness with a cake tester or skewer
inserted just off-center in the cake. If it comes out
with wet batter clinging, it needs more time in the
oven. Many factors affect the exact time each cake takes
to bake, so don't simply use the time recommended by
the recipe. You, the cook, are the best judge of when
your cake is done.
Occasionally cooks assume that they can reduce the amount
of sugar or fat in a cake recipe to save on calories.
However, both these ingredients have a strong influence
on the texture and volume of baked products, so you
can't arbitrarily mess with sugar and fat proportions
and assume that the cake will be the same. Sugar also
affects the temperature at which the cake sets, so changing
proportions also changes baking times.
Perhaps, too, your baking powder is old and no longer
effective. Baking powder keeps for about a year, providing
it's stored with the lid on tightly. You can test its
effectiveness by dissolving one teaspoon in a cup of
hot water. If lots of tiny bubbles form and rise, it's
still working. If not, replace it.
We'll bet that one of the above reasons applies. Wishing
you lofty cakes that bake as they should!
Anne and Sue