I recently bought a yogurt maker, and have used it about a half dozen
times. I'm having a problem reusing one batch of yogurt starter for
the next. The instructions warned me that after a few batches the
starter would get "tired," and I would have to start over with
commercial starter. That would be fine, but the starter is not
lasting for a few batches. The first time I use it the result is
thinner than the original; the second time it's unusable.

I'm following the directions I got, but they're vague. I wonder if
I'm doing something wrong. I put a half cup of each new batch in a
separate clean container, which I store in the fridge next to the
regular container. When I make the next batch I stir in the half cup
just before I put the batch in the yogurt maker.

Any ideas?