I’m more experienced with water kefir grains but maybe some of my experiences will help you.
With water kefir grains, when you first get them, they generally need a rehydration process to activate them fully……. I’m assuming it’s similar for milk grains?
Another thing is if I leave my water kefir grains for longer than 48 hours to ferment, they start to take on a weird smell because water kefir will eventually turn to vinegar like substance if left too long. So maybe that happens with milk kefir? You get kind of an off smelly milk if left too long?
You also don’t want to get into a habit of leaving your grains in too long because that can starve them and hurt them.