bad yogurt

how come my yogurt efforts keep failing? that's twice now.

first batch mistakes:

1. too little existing yogurt to add
2. existing yogurt too old
3. allowed the milk to cool off too much

second batch mistakes:

1. no idea.

it's watery, curdled and extremely separated. a bit like i tried to make paneer. maybe there was some vinegar or other acidic thing on my pan. i used the red le cruset pan to heat the milk and to put the yogurt to bed in.

what's up with this? i used to have no problems at all!