I'm not sure on that. If you think your thermometer is a bit off, then aim for blood temp, or slightly warmer. Did you insulate the container really well? It needs to cool extremely slowly to get the cultures to grow. Aside from that, not sure, I'm sorry. You could try just making with half a litre or less of milk, and reduce the starter yoghurt amount accordingly, if you want to have a success before using lots of milk.