Suitable environment, mainly temperature.

The suitable medium is milk.

purebred

All three are indispensable.

If there is no problem with milk, it depends on whether the fermented things, that is, seeds, have expired and inactivated lactic acid bacteria.

I want to know how the yogurt machine works. Buy a box of yogurt instead of seeds. Just try it. If the fermentation is successful, it means that your seeds are not working.